Do We Have This Trope Already? Exactly What It Says On The Tin, it's the opposite of Hey Its That Guy; It's when you watch a movie and you see an actor playing a character, and think the actor is a particularly famous or recognizable one, but you learn later on that it was some other actor.

Examples:

When I saw Gettysburg, I thought General Pickett was played by Tim Curry, but I learned later on that Pickett was played by Stephen Lang.