An effective way to build convincing characters is to show them in action. Not necessarily physical action. Emotional, mental, and social actions work too.
For emotional and mental actions it may be best to get inside the character's head. But you can show the external symptoms of emotional struggle or mental calculation. That's the way it's done in movie scripts.
____________________________________You can tell readers about a character, but this technique is emotionally "cool": less involving. It does have the advantage of being compact, but I use it more as a supplement to showing action.
____________________________________Another effective way to portray characters is give them family, friends, co-workers and side-kicks, frenemies, and outright enemies.
In "The Super Olympian" especially I worked to show the interactions of my sort-of superheroine with her brother, two sisters, and a few intimate friends who are almost family, and mention a few times how she was influenced by her mother.
In "Shapechanger's Birth" my heroine has no family, but makes a few close friends into a substitute family.
In "Sea Monster's Revenge" my heroine is a marine biologist accidentally turned into a were-seamonster. She befriends two "tribes" or cohorts of dolphins, talks to them in sea monster form, and invents a translation device which lets dolphins talk to humans.
____________________________________What about you? Are there any characterization techniques you think especially effective?