JJ Abrams announced that Zachary Quinto from Heroes has been cast as Spock and that Leonard Nimoy will also appear as Spock in the film. This is a big deal since Nimoy hasn't accepted an acting gig in several years, so this must be more than a just a cameo.
They haven't cast Kirk yet and Abrams asked the fans for suggestions. They are also trying to work William Shatner into the film, but Abrams said it had to be a part worthy of him.
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