Complete reevaluation
Sometimes a movie or show comes along that shows you: hey, this person is actually an actor, they're actually great.
I am watching "Extremely Wicked, Shockingly Evil and Vile." From the moment Zac Effron appeared onscreen he is Ted Bundy. He is nailing this character and has completely transformed. I have a new and serious respect for him as an actor.
Have you seen it? What movies have fully converted you into a fan of an actor you previously disliked?