thats what the games, tv show and EU are for! Lucas is covering his back financially! He needs to keep us guessing, and plugged into his universe.
Plus there isn't very much screentime when we can see Anikan going pure darkside evil, which is why most of EP 3 he'll be corrupted anikan rather then redeemable vader. Plus vader has 3 movies worth of kicking the crap out of the light side, i want to see anikan unleashing some anger. 😎
http://www.theforce.net/episode3/jtf/anakinskywalker.shtml
James Earl Jones talks about his role being about 5 minutes in length, "right after Anakin goes bionic and falls into a volcano." - JEJ
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and thats just dialogue, perhaps more scenes with vader present.
plus that quote was from before the reshooting, and i believe jej was called in for additional dialogue, so who knows?
certainly well over 2 minutes though