Benicio will be acting with Glenn Close, Chris Pratt, Zoe Saldana, and John C. Reilly in the sci-fi action film. It is assumed several of the characters will be CGI, considering one of them is a anthropomorphic raccoon and the rest are various types of aliens.
More exciting, possibly, is the rumor that del Toro will be playing Thanos, who was teased at the end of The Avengers as the main bad guy. Again, considering Thanos is a hulking alien bent of world destruction, he would only be putting his voice behind the role. It would also mean that del Toro would presumably sign on for Avengers 2, which comes out a year after Guardians of the Galaxy's 2014 release.
Marvel has been snapping up stars left and right for the next batch of films based on their properties. Robert Redford will star Captain America: The Winter Soldier, and Anthony Hopkins and Christopher Eccleston have signed on to return in Thor: The Dark World.