It takes a special talent to deliver some of the dialogue that Lucas comes up with in an engaging and believable fashion.

At no point do I stop believing in the characters in ANH and ESB and a lot of that came from very good casting (Lucas himself has admitted to not really being an actors director).

Even in ROTJ most of the time the original cast and Ian McDiarmid manage to pull it off even without help from the directors (Marquand and Lucas by most accounts).

By the PT even the most talented actors (besides Ian who only drops the ball when the Halloween mask is put on) seem a bit lost and Hayden and Jake (playing the most important character in the PT) don't seem to have a clue what they are doing.

Redubs could help as so much is conveyed with the voice.

I was surprised by how good Anton Yelchin was in Terminator 4 (especially seeing how awful he was in Star Trek) I really believed he could grow up to be the same character Michael Biehn played in the first film.

Nico Cortez also did an amazing job as young Bill Adama in Razor.