” I count Christopher Lee (Count Dooku) and Ian McDiarmid (Senator/Chancellor Palpatine) as the great exceptions. These two talented actors have been around in the industry long enough that they can turn in a great performance even with bad direction or poorly written dialog. I think both steal every scene they are in.”
I agree, they delivered solid acting in every scene they were in, no matter how poorly conceived or written the scene was... but they were by far the exception in these films.
I was re-watching Space: 1999 a few months ago and was surprised to see how many British actors made appearances as guest stars who would go on to be cast in Star Wars movies: Christopher Lee, Peter Cushing, Julian Glover, Brian Blessed, David Prowse.