No one ever found wisdom without also being a fool. Writers, alas, have to be fools in public, while the rest of the human race can cover its tracks.
You want to try and bring a character to life in an honest a way as you possibly can. It doesn't matter whether he's a doctor, an actor, a car salesman or a captain of a starship. If you can bring truth and honesty to that character, then your audience will believe you.
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.