Lajos Egri's argument, a century old and still unbeaten, is that character is not a list of traits — it is a dominant drive under pressure. A person defined by one ruling passion will do things a plot outline would never have thought to make them do, and that is where drama stops feeling manufactured.
The generator is contradiction. A character built from two incompatible drives produces conflict without anyone forcing it: the loyal soldier who cannot follow an order he believes is wrong, the devoted mother whose devotion is really control. Every scene can now ask which drive wins today.
Contradiction is not the same as inconsistency. Inconsistency is a character behaving differently because the scene needs it. Contradiction is a character behaving differently because a stable internal conflict tipped the other way, and the audience can feel the tip.
The test of a real contradiction is cost. If the character can hold both drives without ever losing anything, you have a quirk. Find the scene where holding both is impossible, and you have found what the screenplay is actually about.