When you forget that they're an actor and think of them as the character they're playing.
Alfred Molina made me so mad in *Not Without My Daughter* that I wanted reach through the screen and choke him!
And I know I'm not the only one, Molina was on the Jimmy Kimmel show promoting *Spiderman 2* and Jimmy asked him *Do people recognize you?* He said *Yes, this man walked up to me and said 'Are you the guy from No Without My Daughter?' I said Oh, a fan, why yes, yes I am! ... and WHAM! He punched me right in the face!*
Acting does take experience; the more you do it, the more you encounter onstage, the more theatrical experiences you become familiar with. I think that if the person displays an aptitude to play a open range of a character they become a good actor or actress.