I read the script and try not to bring anything personal into it. I make notes, talk to the director and we decide what kinds of shades should be in the character.
When a performance isn't working, it's usually because the actor is trying to do something and they're not able to express their idea very well. It's a muddled expression.
But there's a difference between having artistic interests and being psychotic. That's more than a fine line of differentiation, and I do see that a bit too much.