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Source: https://writers.stackexchange.com/a/47909
License name: CC BY-SA 4.0
License URL: https://creativecommons.org/licenses/by-sa/4.0/

Source: https://writers.stackexchange.com/a/47909
License name: CC BY-SA 4.0
License URL: https://creativecommons.org/licenses/by-sa/4.0/

The only technique there is really is keeping some sort of "character sheet" for each character.

If you can keep them all in your mind, that's great, but I guess you wouldn't have been asking the question if it were that easy for you. Otherwise, people use different platforms. I like OneNote, I've seen others use Scrivener, or physical folders with actual paper, some do just keep it all in their heads - whatever works for each writer.

You want to keep track of stuff related to each character: appearance, personal history, goals, views they express. The big stuff, like what kind of person they are - you're not going to forget that as a writer. But little details like eye colour or a food allergy - those are easy to mess up. They're not really important, so you write something in one scene, then forget about it and write something different next time the issue comes up. As an example, the character Thomas Raith in Jim Butcher's _Dresden Files_ series is just under 6 feet in some books, a little over 6 feet in other books. When this was pointed out to the author, the next book mentioned how the character wears shoes with whatever heels are fashionable at the moment, so the first-person narrator isn't really sure about his real height.  
The way you avoid this is you try to double-check such things on the character sheet you're keeping.

To clarify, you don't need to create all this stuff in advance. But as you're writing, make notes of what you're writing about the character, so you don't contradict yourself later. (And of course, if there's something you don't remember, such as whether you've already killed a particular minor character, you can Ctrl+f your file.)

Original score: 3