Horrible acting isn't necessarily the fault of the actor. I remember an interview with Ewan McGregor where he recounted being in front of a green screen, sat on a green saddle that was mechanically rocking back and forth (that would be CGIed into an animal of some sort). George Lucas kept shouting "look at the MOONS!", to which McGregor replied "WHAT MOONS?".

TL;DR: actors are only as good as the environment you give them to work in.