When I write, I take my cues from screenwriting, except I’m the actor. Since internal dialogue doesn’t convert easily o the movie screen, I tend to develop most of the characterizations within action. I step through the scene in my mind and react to the events as my characters would, physically and mentally. I can do that because I know their personalities well enough to know what they would do in a given situation.
I know them because I create backstory/biographies for each of them. I want my readers to identify the character more by what s/he does and says rather than what I might tell them (aka author intrusion).