Okay, let's break this down logically. All of the advice bits you are mentioning are from the 1980's. In the 1980's -- a time of many large books -- it became a popular school of style to promote minimalist writing -- where the narrative is as close to a film/t.v. screenplay as possible: minimal description, minimal poetry, imagery or metaphor, minimal exposition, trying to shove almost all information into dialogue with minimal dialogue tags and plenty of snappy patter, emphasis on violent and stunt action, short chapters and sentences, minimal to no adverbs, no third person omniscient viewpoint format, show don't tell, etc. -- all of the ones you most commonly hear bandied about even today came from this particular school of writing stylistic preferences that became popular in the 1980's. It was a style particularly pushed for suspense fiction -- mysteries and thrillers -- where it could work very well. There have been a number of highly successful thriller writers, some still alive and producing, who naturally have that style (and sometimes also did screenplays,) and if that style interests you, they are worth studying: Robert B. Parker, Carl Hiaasen, Elmore Leonard.
Were most of the novels written in the 1980's short and using the minimalist style? Absolutely not. Minimalist, action-focused stuff was a slice of the market, with no more or no less chance of best-sellerdom and being made into movies than other types. The biggest authors of the 1980's all pretty much wrote large novels, though they also wrote short fiction, there being still more of a magazine/short fiction market left then -- Stephen King, Judith Krantz, Tom Clancy, James Clavell, James Michener, Danielle Steele, etc. But movies were considered much higher status than novels at that time, so a lean and screenplay like style sounded good to a lot of people and attributes of that style were then passed around as writing guidance, declared necessary for writers to follow because that always makes the advice sound more authoritative and expert.
Science fiction, fantasy, and SFF horror stories are primarily suspense fiction and a lot of the time have a fair amount of violent action, often being war novels or adventure stories. So naturally, that idea of a minimalist style worked its way into SFFH too. But SFF stories have to, sometimes quickly and sometimes in more complicated fashion, establish an entirely different story universe, even if it's near future SF or contemporary fantasy. Those stories tend to require more framing and scaffolding, with exposition and description, than most realistic stories, to convey the info about the new universe. So while the minimalist style can work in SFF, especially in short fiction, it tends to be even less of a slice of the SFF market and isn't most of the time a natural fit for the personal writing style and interests of SFF authors. Publishers were being able in the 1980's, for a variety of market and distribution reasons, to do longer books more often and SFF authors, no longer as often originating out of and training in short fiction in the magazines, and with much of their stuff highly unlikely to ever be dramatically adapted, quite often went for stories in bigger, more epic scope, particularly in fantasy. And they sold. If you didn't do that, you also could sell -- the market likes variety and that includes a variety of styles and lengths.
So if the minimalist style was never dominant, even in its heyday, why is it still so persistent thirty some years later in advice to writers? There are a number of reasons, such as: 1) we still prize film/screenplays more than novels in the general society, so anything that sounds movie-like sounds good; 2) it's a way of distinguishing current material from past classics -- even though many past classics were short, some of the best known are large and so it was a way of saying the narrative was "modern" by not being Victorian-ish; 3) the advice got enshrined in writing advice books, many of which have simply been revised and reissued over the decades, and so it's simply become folk wisdom that people hear and repeat; 4) newer writers are believed to be mainly awful at many things and it's assumed most of them will meander and lack focus, so the bromides of the minimalist style are an easy set of instructions; and 5) many established writers tend to repeat the advice because it's been around for so long, even though they themselves don't really follow it, and if it comes from a high-selling author, it must be gospel to some writers who then further repeat it, etc. You also have fans who really do like a more minimalist style and who complain, loudly, about popular books that don't have it, which is most of them, so that tends to give the impression that it's a crushing issue, when it's really just an issue of stylistic preferences and the author's own personal style and goals for each story.
Mainly, the reason that the minimalist catch phrases have hung on is that authors are pretty much open to embracing any set of "rules" that is sold to them as a guarantee of wide approval and success getting published and selling -- the magic key, where there is one true way to write and it is the good writing and it is the golden ticket. It's understandable in a market as wide open and chaotic as fiction publishing that people would want that, (which is why I try not to take it personally when I get snarled at for pointing out that books exist in huge numbers that refute what is being toted as the true writing gospel.)
"Kill your darlings" is less of a minimalist one than one that simply says, watch what ticks you have as a writer in your prose choices. It came from Faulkner and had a burst of popularity in again the 1980's thanks to Stephen King pushing it as Faulkner's advice. Writers will tend to repeat words or do little things stylistically that come naturally to them and which may or may not be needed to be weeded out in revision. George Martin has put repetition and his ticks to good effect mostly (winter is coming
it's something that may work, or the writer may want to catch and change it. But it's not really a cutting length issue.
"Cut the first few pages of your novel and start when the action starts" is a stylistic preference about openings and it is a minimalist trait, though many minimalist stories start with a conversation, rather than straight violent action. It's a type of opening that can work, or might not, depending on what the author is doing in the story and what sort of universe the story deals with. It's an opening that works better in SF or contemporary fantasy, where many things are already familiar, than in secondary world fantasy, but it's not unworkable in any SFF stories. But it doesn't necessarily fit the style, focus and approach that every story needs. A lot of readers like to be lead into a world first, get to know the characters, and then go into the violent action, often contrasting with the peacefulness of the world before that action starts, creating strong emotional drama at the loss, as Tolkien did with Lord of the Rings. (Or contrasting one world, usually our normal everyday world, with the discovery of another world that is very different, in both fantasy and alternate reality or space opera SF.) Tolkien was in fact using a very standard British writing style, the sort of fable-fairy tale style where you first introduce your readers to the framework you'll be using and readers can hear the plumey British voiceover narration in their heads. (J.K. Rowling also uses this style.) That style still works today, in its American forms, etc., as well. It's called a build-up. They use it in movies as well, especially horror. Some writers don't like doing build-ups -- that's not their style. So "start right into the (violent) action" sounds good to them and that's how they tell others to write. But they also are tending to do stories that don't need a build-up to launch into that violent action -- that's not their focus. For other writers, it is, because it is important to developing the story.
"Get rid of anything that doesn't move the plot forward" -- This is a minimalist value, and it's also a value of those who might not be so minimalist but want to center their stories primarily on a chain of violent action and thrills. This is a very cinematic focus, where there's not going to be a lot of inner life. The plot becomes the main focus and its structure and twists are the important part, so everything else is considered excess (minimalist.) This is not, however, what every reader wants out of a story, nor is violent action the main focus of every story. It's a stylistic focus that can work very well, but may end up sacrificing characters as not very memorable or interesting for many. And it's characters that SFF readers actually tend to glom on to as fans, more than just plots and stunts. That doesn't mean that plots and stunts can't be important. But there are numerous ways to do them and all the elements are linked, so again, depends on the story, its tone, and the author's style and goals for it. And in SFF, spending time on things that don't move the plot forward but do build the world of the story to give the plot emotional context and weight, may be essential. It's those sorts of details that many SFF readers love -- bits of science or alien culture or clothing description or old ballads that don't really have much to do with the plot but do make the story richer for many readers. Do I need elaborate descriptions of sword-fighting positions and their names in training sequences to then read a sword-fight? No, I do not but it can be flavoring.
Alchemist said:
Again and again I read reviews and views on classics that they are overly long, "I don't know how the author possibly got this by an editor," or "It took me 200 pages to get into it," etc.
Which tells you the stylistic preferences of the reviewer. Which may be helpful if you have the same preferences, but otherwise, probably not. This is largely why authors should not freak out about reviews.
And more specifically, how does one decide what should and should not be cut from a manuscript? Or do you just trust your agent or editor, and hand in everything and let them take the scalpel to your hard work?
Since many agents do not perform editorial functions and many editors don't have much time to edit your work, and in any case, are not in charge of your book, that would seem unwise. For instance, Jo says that Teresa "made" her put four chapters back into the book. This is not literally true of course. Teresa can't make Jo do anything. But Teresa did point out what she thought were possible problems and made suggestions. And Jo evaluated that, decided that Teresa's comments were valid, and decided to put the chapters back in and have Teresa look at them. So Teresa helped and probably tried to persuade where she thought it important; Jo decided. The only one who can figure out what really needs to be cut or not from the story is the author. Agents and editors don't "fix" manuscripts or write it for you. If that was the case, they wouldn't need you at all. They take you on in the first place because they already feel you've written something they like. They'll try to catch where you've made mistakes from what you wanted to do, inconsistencies, areas where it seems murky. But it's your creation, what you express, what you want to say to readers. And your instincts about what to cut and where.
Some authors cut mentally and edit as they go, ending in little revision and probably more stuff added in than taken out. That's their process. Others must meander, write reams of stuff they then don't actually use, make elaborate character and world bibles to keep track of details even if they don't use most of the material. But they needed it to figure things out. That's their process. No matter how helpful an editor is, they can't hot-wire your brain differently. You're your own guide.
And you don't trust yourself to do that and that's normal. You never will completely trust yourself. Stephen King and J.K. Rowling and Margaret Atwood all don't completely trust themselves. But they try. And as you've already noted, the marketplace has a wide array of styles, lengths, etc. So looking at what works for you will not box you out.
Should the writerly advice be trusted, or should we look more to the actual books?
Writerly advice can be mined. It's not a matter of trusting it or not because it's not a right or wrong situation. It's just suggestions, usually stylistic ones. If the advice works with your writing and your brain, it can be helpful. If it doesn't, it is not going to help you because you won't be able to understand it and/or implement it. Especially since most of said advice again tends to be saying that there is one writing style preference to rule them all, which there is not. You're searching for feedback that is helpful, not instructions. You are not assembling a piece of IKEA furniture.
The books you like and feel are close to your style and type of story will be the most useful to you. You can look at how they do what interests you about them, as we've discussed before. You won't know what they've cut from it, of course, but you can look at how they do pacing, chapter structure, etc. and then you pick and choose what technique might work for what you are doing. If you can find an article or interview where the author is talking about how they developed a work, rather than trying to dispense writerly advice to all, I find that those are the most helpful to people. One thing that might be useful is that author John Scalzi has a feature on his blog, Whatever.com, that is called The Big Idea, where he lets other authors promote their works by talking about the big idea for the work and how it developed. That's not down in the nitty gritty, but it may give you ideas for developing plot and what you want to focus on. Stephen King's On Writing is very disappointing when it comes to "writerly advice" but when he talks in the bio part about how some of his works developed, it's helpful and he has an appendix where he goes over a piece of narrative he wrote and how he thought about it, revised it, cut and added, etc. so that might be helpful.
But mainly you are going to have to stop looking for magic keys and start trusting yourself. Editors, editorially minded agents, beta readers, etc. all can help in later stages if you are looking at a cutting issue, but the idea is that you are cutting words that are not what you want to say to the readers, that may be nice but it isn't the material or the scenes or the sound that you want in those spots.
