Is Vin Diesel Playing Groot?

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Is Vin Diesel Playing Groot?
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Vin Diesel posted the above image of Groot on his Facebook page with no context or accompanying message. He's previously hinted his involvement with Guardians of the Galaxy at ComicCon and posted a photo of himself at the Marvel office. We know he's involved with Marvel somehow, but in what role remains to be seen.

Would you like to see Vin Diesel as Groot?