Not to defend the actors on the PT, but GL does several takes of a scene and decides afterwards what is going to be used in the movie. In fact, he admits to this being his favorite part of movie making. He likes to play around in post with what the actors have done during principal photography. In this manner he has tried to 'fix' some of Jake's acting in Episode 1 (I remember him saying something along that line in the Making Of Episode 1 documentary).
If an acting performance is 90% spot on and GL dicides to use the other 10% (takes that you as an actor weren't happy about), how much are you to blame? Remember, the actors aren't present during post and may not even see their perfomance until the trails or the premier.
Besides, let's not forget that on Episode 2 and 3 there are no real sets. Just blue or green screens where sets are going to be added digitaly afterwards (and they somehow these digital sets 'feel' fake). Not much 'help' for the actors there to get into character. Just dots on a screen or wall to get their eye lines right and GL 'famous' direction: "Faster, more intense".
No wonder that, in my opinion, all actors look bad in the prequels. Without any exception, no matter what their track record is.