Update: Following the weekend's Facebook mischief, Vin Diesel has elaborated on the Groot business (although there's still no actual mention of that actual name) and his dalliance with Marvel. So why a tree? "As an actor not everything you do has to be the obvious choice," he told the Press Association in Hollywood, "and sometimes the best thing you can do is defy expectations. If everyone expects you to go for this one thing [likely a reference to the popular assumption he'd be Thanos] and you flip it entirely and go for the strangest Marvel character, it's interesting. And when it's interesting it's inspiring."
Also inspiring to Vin was the concept artwork he's been shown, but it seems the clincher was the size of the role, allowing it to be slotted into his busy upcoming Riddick / Fast 7 schedule. "The audience wanted Vin and Marvel, but I'm too busy to do a six-month role," he says. "What Marvel came up with was really interesting..."
Vin Diesel has somewhere in the region of 45 million friends on his Facebook page, and enjoys nothing better than a gradual drip-fed news reveal. A couple of months ago he teased that he was going in to talk to Marvel about something secret, and in only-marginally-oblique style he appears to have let the tree out of the bag. A temporary change of his profile pic to one of an Ent-looking wood man would seem to indicate that he's to be Groot in Guardians Of The Galaxy. "I am Groot," he seems to be saying... and that's basically all he'll be saying if he has the role, since that's about the limit of the character's vocabulary.
Possibly the last of his race, Groot is an alien plant monster who can regenerate himself from twigs. So he's a bit like Swamp Thing, if Swamp Thing worked for S.H.I.E.L.D. (or rather its space equivalent). In the comics he's battled Hulk and Thanos, and has a touching affiliation with Rocket Raccoon. As we said, his sole line is "I am Groot" (although he gets a lot of expression on those words), but then we've only heard movie-Hulk say "Smash!" and "Puny God" so far, and we were fine with that.
Assuming this is what's going on, it would not be Diesel's first turn as a taciturn animation, since he was Brad Bird's The Iron Giant back in 1999, which gives him voice-acting kudos for the rest of time. While it might sound like an easy gig for an expensive actor, just think of all the different nuances he can bring to this performance. "I am Groot." "I am Groot." "I am Groot." I am Groot!" He's probably practising with a tape recorder right now.
We are, of course, still awaiting official confirmation on Groot's voice, and that of Rocket Raccoon - and there remains the question of Thanos, who's due to appear in the film in some (baddie) capacity and has also yet to be announced.
James Gunn is the Guardians director, and the rest of his cast includes Chris Pratt, Michael Rooker, Karen Gillan, Zoe Saldana, Benicio Del Toro, Glenn Close, Lee Pace, Dave Bautista and Ophelia Lovibond. Shooting is underway right now, for a release next summer.