Bradley Cooper is the actor most associated with Guardians of the Galaxy's Rocket, by virtue of the fact that he provides the character's voice. However, it's another actor who actually does a lot of the work to bring Rocket to life. Sean Gunn, the brother of Guardians director James Gunn, and the man who plays the role of Kraglin, plays Rocket on the set in a green screen body suit while in a permanent crouch. Considering what Gunn puts his body through, he'd be forgiven for being a little put out that he doesn't get a lot of the credit, but the actor says he doesn't do it for the credit. According to Sean Gunn...
It is strange to spend so many hours as a character and then see it and have it be neither my voice nor my face. But I never did the job because I thought it would get me a lot of attention. I did the job because I wanted the movie to be as good as it could possibly be.
Sean Gunn may not mind that he doesn't get the recognition, but the fact is that he should get a lot of it. Voice acting is a much harder job than many people give it credit for, so Bradley Cooper shouldn't be overlooked, but the two are, at the very least, equal partners in bringing Rocket to life on screen. Sean Gunn doesn't simply give Rocket a place for the CGI effects people to focus, but he also aides the other actors on the set by giving them somebody to actually act with, rather than having to emote toward empty air or a tennis ball on a stick.
Sean Gunn had portrayed Rocket in both Guardians of the Galaxy movies but Entertainment Weekly reports that he did not assume he would have the same role in Avengers: Infinity War, as it was being directed by Joe and Anthony Russo rather than his own brother. The Russos apparently didn't hesitate to put Gunn out there with the other actors, which likely helped everybody who had to interact with Rocket on screen.
We'll see Rocket along with the rest of the Guardians of the Galaxy when they unite with the rest of the Marvel Cinematic Universe for Avengers: Infinity War when it hits theaters in just a few weeks. While you watch it, try to remember that behind that CGI "not a raccoon" there's a guy in green spandex doing his best to make the character come to life. He may not do it for the credit, but he still deserves it.