Peyton Reed was considered to direct the movie before James Gunn was hired.
Joel Edgerton, Eddie Redmayne, Jensen Ackles, Jack Huston, Cam Gigandet, Sullivan Stapleton, Logan Marshall-Green, Garrett Hedlund, Chris Lowell, James Marsden, Jim Sturgess, Joseph Gordon-Levitt, Aaron Paul, Michael Rosenbaum, Wes Bentley, John Krasinski, Lee Pace and Glenn Howerton auditioned for the role of Star-Lord before Chris Pratt was cast. Howerton would reportedly be offered the part if Pratt hadn't won it.
Gunn originally didn't want to watch Pratt's audition tape, believing him to be wrong for the role, but was ultimately convinced by his assistant and immediately decided Pratt was the ideal Star-Lord.
Gina Carano, Olivia Wilde, Rachel Nichols, Thandie Newton and Adrianne Palicki auditioned for the role of Gamora, while Elizabeth Banks campaigned for the part, before Zoe Saldana was cast.
Jason Momoa was offered the role of Drax, but turned it down due to salary issues. Djimon Hounson, Brian Patrick Wade and Isaiah Mustafa also auditioned before Dave Bautista was cast.
Jim Carrey, Adam Sandler and David Tennat were approached to voice Rocket before Bradley Cooper was cast.
Gal Gadot auditoned for the role of Nebula.
Early drafts featured Richard Rider becoming Nova as a counter-point to Star-Lord. In further revisions, Rider was cut so Star-Lord could be the sole focus.
Early drafts featured Bug as a member of the team, but he had to be cut due to copyright issues.
Early drafts featured an appearance of Adam Warlock.
Ronan's soldiers were originally the Badoon, but their rights were with 20th Century Fox, requiring them to be changed to Saakarans.
Gunn originally didn't want to include Thanos in the movie as mandated by the studio, as he believed it'd cheapen Ronan's role as the main villain. However, he found an appropriate middle ground in Ronan threatening to go after Thanos and killing his lackey, the Other.
Early drafts included the Spartoi and the Nova Centurions.
In early drafts, Yondu would die during the Battle of Xandar.
Early cuts featured additional scenes set to "Livin' Thing" by ELO and "Spirit in the Sky" by Norman Greenbaum. Also, "Hooked on a Feeling" was originally set to Star-Lord's arrival at the Moragian temple to retrieve the orb.
Early cuts featured an extended version of the epilogue set to "Ain't No Mountain High Enough", revealing an elderly Grandpa Quill still hoping Peter will return one day. The segment was cut because several test-audiences didn't recognize actor Gregg Henry due to the prosthetics used to make him look older, and felt that the scene was too depressing.
Marvel wanted to use Galactus and the Silver Surfer in the scene where the Collector is explaining the Infinity Stones, and offered to extend the deadline for the rights of Daredevil so Fox could complete a problematic R-rated movie written and directed by Joe Carnahan that they were producing, in exchange for letting the Fantastic Four and their affiliated characters reverse. Fox preferred to hold on to the Fantastic Four, and a Celestial was used in that scene instead.