There’s some great advice here for actors, both in your own performance or in watching/evaluating another’s performance.
That said, as a writer, I think there’s an awful lot to be said for some of these observations. The ability to reveal yourself on the page, to make your characters face uncomfortable situations and honestly deal with truths is what will define a good character and by extension, make you a better writer.
Mostly, I’m posting this for me to remember. One of the hardest things for me is to be cruel to my characters, to let them not be nice people, to put them in circumstances where the easy thing to do might be the least honorable.