When Dave Bautista found out he got the role of Drax the Destroyer, he broke down in tears, overjoyed at getting a Marvel comic book role. He prepared for the role with an acting coach.
Vin Diesel recorded all his lines in several different languages, including Russian, Mandarin, Spanish, Portuguese, German, and French, so that they could use his real voice in the film around the world.
(at around 56 mins) The scene where Peter drops the orb during the Collector scene was not scripted. According to the commentary, Chris Pratt accidentally dropped it during filming, but remained in character through the whole thing, so it stayed in the final film. Chris Pratt later revealed to James Gunn that he intentionally dropped the orb, figuring it's better to ask for forgiveness than permission.
Chris Pratt apparently stole his Star-Lord costume from the set, for the sole purpose of having it available so he could show up in costume to visit sick children in the hospital, who might want to meet Star-Lord.
James Gunn stated that Chris Pratt's audition was so good, he was prepared to offer him the role, even if Pratt did not lose weight, or get in shape in time. Gunn joked that he was willing to CGI a six-pack on Pratt's body. However, Pratt asked Gunn to give him six months to lose fifty pounds, and he ended up losing sixty.
Stan Lee: (at around 16 mins) As one of the Xandarian citizens Rocket scans while looking for a bounty. Lee got this cameo because although he did not create the Guardians of the Galaxy, he is the official co-creator of the character Groot, along with Jack Kirby and Dick Ayers.
Rob Zombie: Ravager navigator. Zombie always appears in movies by James Gunn albeit in voice cameos only.
Lloyd Kaufman: (at around 25 mins) As Star-Lord, Gamora, Rocket, and Groot walk into the prison, they are greeted by a row of angry inmates standing on a balcony; Kaufman is at the left of the screen, shouting "Murderer!" at Star-Lord.