As the prequels are set decades before the other films, being familiar with the OT ought not to be a major requirement to playing a part in them. (Obi Wan notwithstanding.) Either the character is there on the script page or it isn't.
Aren't we all forgetting a certain actor who wanted to tie George to a chair, and force him to say those lines back in 1976? Carrie Fisher probably still has nightmares about "faster, more intense". ;)