James Gunn, director of the much anticipated Guardians of the Galaxy, has been speaking to Total Film about the techniques used to bring Rocket Raccoon (who’s set to be voiced by Bradley Cooper) to the screen in the cosmic Marvel movie. On set, Gunn made sure they had a real raccoon named Oreo in order to maintain a sense of reality during filming, “we needed a raccoon to study how he looked and his behavious, so that our on-screen raccoon, which is generated through CGI is realistic.”

Bradley Cooper wasn’t the only contribution to Rocket Raccoon, with the director’s brother Sean also standing in for the character on set: “Our Rocket is based on a combination of our voice actor, Bradley Cooper, our on-set actor, my brother Sean Gunn, the movements and behavior and look of Oreo, as well as my own animation… Bradley is one of those actors that can do drama and comedy equally well. He can do character work as well as being a leading man. These are important qualities in the role of Rocket – which is a lot more dramatic than you might think.”

Guardians of the Galaxy opens August 1st 2014, with a cast that also includes Chris Pratt (Zero Dark Thirty), Zoe Saldana (Avatar), Dave Bautista (The Man with the Iron Fists), Vin Diesel (Fast & Furious 6) Glenn Close (Damages), Karen Gillan (Doctor Who), John C. Reilly (We Need to Talk About Kevin), Benicio Del Toro (Savages), Djimon Hounsou (Blood Diamond), Michael Rooker (The Walking Dead) and Peter Serafinowicz (Shaun of the Dead).