Here is Eliezer Yudkowski on
How to Write Deep Characters:A good rule of thumb is that to create a 3D character, that person must contain at least two different 2D characters who come into conflict. Contrary to the first thought that crosses your mind, three-dimensional good people are constructed by combining at least two different good people with two different ideals, not by combining a good person and a bad person. Deep sympathetic characters have two sympathetic parts in conflict, not a sympathetic part in conflict with an unsympathetic part. Deep smart characters are created by combining at least two different people who are geniuses...
Conflicts between evil and evil are even shallower than conflicts between good and evil, which is why what passes for 'maturity' in some literature is so uninteresting. There's nothing to choose there, no decision to await with bated breath, just an author showing off their disillusionment as a claim of sophistication.
I never thought of that.
