The most embarrassing moment I've had was Heath Ledger. I thought that Nolan was off his rocker when he cast him as Joker. Goddammit was I wrong.
I didn't like Robert Pattinson. Up to and including Remember Me, I thought he couldn't act his way out of a paper bag. Then he took the role in Good Time, and I realized that his shortcomings were script and direction problems. The man is damn good.
Most recently is Zac Efron.
He always struck me as the low-brow, Disney boy who makes girls' ginies tickle without much talent.
Iron Claw changed my mind about him.
He's a good actor.
He put his life, soul, and body into that role, and he was great.
I've learned that I need to understand that great actors often work with bad directors and bad scripts, and that they're people. It shouldn't be held against an actor when he takes a role for a paycheck, when he's nineteen, and is simply taking the job.