According to Comic Book Movie on Saturday, on James Gunn's Facebook page, he clarified that Thanos indeed will not be played by "Fast and the Furious" actor Vin Diesel. He emphasized that he'll only be voicing Groot in "Guardians of the Galaxy." What instigated this reiteration was the fact that fans had been asking questions on Gunn's Facebook page, so he saw to it that things were straight with the general audience.
For those who may not have seen Marvel Studios' "The Avengers", Thanos was the "man behind the curtain" in the mid-credits scene of the film. Chitauri aliens had invaded Earth and the encounter was deemed The Battle of New York where Agent Phil Coulson lost his life at the hands of Loki and was once again reunited with his peers on "the bus" in ABC's "Marvel's Agents of S.H.I.E.L.D."
The Chitauri had failed and at the end of the film Thanos was pondering what to do next when it was revealed to him that the humans still stand. Upon being told that continued encounters with the Earthlings would be comparable to that of "courting death" he turns, revealing his purple, hideous face, and grins. In case one may be wondering why he's grinning, there's a whole back story referring to Thanos' obsession with Mistress Death and the insane measures he takes when attempting to please her.
At that time, Thanos was played by actor Damion Poitier, but now the villain has been casted by an unnamed actor under the announcement of Marvel president Kevin Feige. When Gunn was approached about who was playing the villain, he would say no more than, "You'll have to wait and see. BUT IT IS NOT VIN DIESEL. He is Groot. Speaking of Vin...." If Diesel had been in the vicinity, chances are he would've been coerced into telling Gunn, "Hey! That's my line!"
The only thing that is known about Thanos is the fact he's a supporting character in "Guardians of the Galaxy" and this will be the set-up for him for the "Avengers 3" film. Clevver Movies did report on their YouTube channel a back and forth dialogue describing Thanos' scene in the film and someone who is familiar with the character's environment, would be able to do the math and go from there.
"Guardians of the Galaxy" is set to open in theaters on Aug. 1.