Executive Meddling: Starhawk was supposed to be turned evil and killed off, but the editors wouldn't let Valentino. His departure allowed Michael Gallagher (who did like the character) to keep him around and redeem him.
DnA for the creative team behind the 2008 series, Dan Abnett and Andy Lanning.
Gotgino (Guardians of the GalaxyIn-Name-Only) for Bendis run on the series.
What Could Have Been: Jim Valentino has an entire thread here for this Trope. Essentially: Starhawk and Vance Astro would both die, the team would fight Kang and several of their groups of enemies (who would be redesigned to turn them into even more blatant Legion of Super-Heroes analogues), the Protege would turn out to be the Beyonder, and by the end of Valentino's planned run, the group would morph into a Legion of Super-Heroes type organization with a roster of over several dozen heroes.
Ability over Appearance: At first. According to Word of God, Chris Pratt had not lost any weight for his audition - but he was just that good. Nonetheless Pratt did insist on losing 60 lbs to prepare.
American Accents: Meredith Quill (Peter's mom) lays the Southern accent on thick. Yondu also has a noticeable Southern accent. Karen Gillan, meanwhile, puts on a very generic American accent for Nebula.
Auteur License: After the seemingly unlikely success of this group of characters, Disney now will be eager to support any adaptation of Marvel Comics characters by Marvel Studios with Ant-Man and Doctor Strange promising to be just the beginning of lucrative new film properties.
Awesome, Dear Boy: Glenn Close was ecstatic to play Nova Prime in, having always wanted to play an M-like authority figure. By all accounts she enjoyed filming the movie and immediately said she was eager to return for a second.
Vin Diesel and Bradley Cooper as Groot and Rocket Raccoon respectively. James Gunn did interview professional voice actors at first before opting for a celebrity one. Played with in that this wasn't too prominently displayed in the marketing.
In the Mexican Spanish dub, Rocket is voiced by the Soap Opera actor Sergio Zurita.
In the Japanese dub, the same character is voiced by the comedian Kouji Kato and Groot by scriptwriter and actor Kenichi Endou.
Rocket, Big Time. James Gunn has stated several times that Rocket was a big, if not the main, reason he wanted to make this movie. In fact when it was confirmed Guardians was a hit, Gunn put a heartfelt thank you letter online and specifically thanks everyone for letting a Raccoon make them a little more human.
Vin Diesel loved his character, Groot, so much that he would go to promotions and premieres wearing the stilts he wore for the movie and a shirt proclaiming "I Am Groot."
Defictionalization: It was one thing to put out the Awesome Mix Vol. 1 (it's another thing that it topped Billboard charts for weeks) but to pull this trope and release on cassette.
In order to play Nebula, Karen Gillanshaved her head completely bald and had to endure hours of makeup and prosthetics being applied before cameras could roll. Her hair was then made into a wig and sent to the Star Wars studios (she got it back to film Selfie). She won't have to go through it all over again for the sequel, though.
Chris Pratt went through an astounding series of physical transformations in the run-up to this film. After spending years as the pudgy Andy on Parks and Recreation he got ripped for Zero Dark Thirty. Then he went back to being pudgy for Parks and Rec before gaining even more weight for Delivery Man. Then he got ripped again for Guardians of the Galaxy winding up sixty pounds less than he was during most of Parks and Recreation. He nearly passed on the project feeling like he couldn't make himself over to get to the body needed for the role, especially after an earlier "embarrassing" audition for one of the G.I. Joe films. But he committed to it and not only got into amazing shape with handsome movie-star looks, he credits it with making his life healthier and better. He is going back to pudgy for Parks and Recreation once again, though, although he says that he is maintaining a stricter exercise regime this time around. He also found Star Lord's mask to be extremely claustrophobic and had to deal with panic attacks while wearing it. The fact that the front and back were bolted together (meaning that it was impossible to take off without assistance) didn't help.
Dave Bautista already had the physique to play Drax (though he did say on Chris Jericho's podcast that he had slimmed down somewhat when he came to Hollywood to possibly play more roles than typical Brutes, so he essentially had to bulk back up for Drax) but he had to spend four hours every morning with his arms outstretched as makeup artists applied and colored the 18 prosthetics needed to achieve the look seen on screen. Also, Bautista was wildly enthusiastic about getting a role in a Marvel movie but didn't have much acting experience so he threw himself into acting classes to prepare.
Vin Diesel decided that he'd record his audio while wearing stilts in order to see the world from Groot's perspective. He said that it was also practice in anticipation of doing Groot's motion capture in any sequels that might happen.
Zoe Saldana had to spend hours getting turned green in order to play Gamora. She said, judging from the strange discharges she kept experiencing, that her body had an adverse reaction to the alcohol-based make-up they were using.
Enforced Method Acting: According to director James Gunn, the translation of Groot's dialogue actually was scripted — but he didn't share it with actors whose characters aren't supposed to be able to understand Groot (i.e. Quill), to get a more genuine response. He did give the translations to Vin Diesel (Groot), and probably also to Bradley Cooper (Rocket).
Follow the Leader: The success of the movie convinced Marvel's rivals to create their own action-comedy superhero movies with anti-hero protagonists. In 2016, 20th Century Fox released the Deadpool movie and Warner Bros. plans to release Suicide Squad (2016). This goes even extends to the trailers (featuring 70s pop & rock music) and even the release dates (both movies were released in traditional dump months just like Guardians).
Gosh Dang It to Heck!: The French dub is globally more polite and kid-friendly than the original version.
I Am Not Shazam: Denarian is Garthan Saal's rank in the Corps, not his first name.
Irony as She Is Cast: Gamora claims she doesn't dance, and does it badly the one time she tries. Zoe Saldana is a professionally trained ballerina.
Mean Character, Nice Actor: Oreo the Raccoon, the raccoon who was used as a reference raccoon to help animate Rocket, is the sweetest and cuddliest raccoon whose ever lived. Rocket, on the other hand, is a bit of a grumpy Jerk Ass.
Who would have though that stone-cold badass Vin Diesel would end up playing the gentle-yet-badass Groot? (Outside of those that watched The Iron Giant, of course — which itself was a role where he was Playing Against Type.)
Korath's lack of recognition for "Star-Lord" reflects the obscurity of the comic series compared to other properties from the Marvel Cinematic Universe. His getting it right at the end shows the hope that the film could raise their profile among fans.
Gamora and Rocket's arguments become a bit funnier when you remember that Zoe Saldana and Bradley Cooper are exes.
After appearing in the trailer, "Hooked on a Feeling" by Blue Swede sold more than 2000 downloads within 24 hours of the trailer being released — a 700% increase in sales.
All of the songs that comprised the film's soundtrack, "Awesome Mix Vol. 1", that was released as an album; it became the first film soundtrack to ever hit #1 on the Billboard album charts without a new song debuting on it. It's also worth pointing out that they were heard on radio stations well after the movie premiered.
Sleeper Hit: In spite of Marvel's excellent box office record, the movie was expected by most to do relatively modest Thor-level numbers of around $180 million because it was based on a very obscure comic book property and was released in the dump month of August. Guardians of the Galaxy, however, ended up getting the last laugh all the way to the top summer spot, dethroning the presumed summer box office champion Transformers: Age of Extinction, and Captain America: The Winter Soldier as the highest grossing movie of 2014. What made its success shocking was that the previous attempts to make big budget sci-fi space operas in 2011 (Green Lantern) and 2012 (John Carter) were massive bombs. The fact that Guardians of the Galaxy was able to avoid the same horrible fate is a real testament to the success of the Marvel Brand.
So My Kids Can Watch: Djimon Hounsou has admitted in an interview that he took the role of Korath the Pursuer for his son, who loves the Superhero genre.
According to James Gunn, Peter flipping off the Nova Corps came from Chris Pratt goofing around on set.
Chris Pratt also ad-libbed the line "if I had a blacklight, this place would look like a Jackson Pollock painting."
The scene where Peter drops the orb during the Collector scene was also not scripted. According to the commentary, Chris Pratt accidentally dropped the orb during filming but remained in character through the whole thing so it's in the final film.
James Gunn's brother Sean, who did the on-set acting for Rocket, improvised the line "A bunch of jackasses standing in a circle."
In 2012, Marvel offered to give Fox an extension on rebooting Daredevil in exchange for the rights to certain cosmic Marvel characters for use in this film; Galactus and Silver Surfer were explicitly named. However, Fox declined the offer.
The song over the opening credits was planned to be "Hooked on a Feeling" but it was decided "Come and Get Your Love" worked better.
There was a rumor about David Tennant voicing Rocket Raccoon. He eventually gets another role in MCU: Zebediah Killgrave a.k.a. The Purple Man in Netflix's TV series, Jessica Jones.
Djimon Hounsou auditioned for Drax and Lee Pace auditioned for Quill.
Early drafts of the script included Bug. He was even back in the Guardians' comic relaunch to promote the character briefly. He was left out because Marvel doesn't own the film rights to the character.
Peyton Reed was in the running to direct Guardians. He would later go on to direct Ant-Man.
Early drafts featured Nova (although it's unclear as to which Nova, Richard Rider or Sam Alexander) as a central character. He was cut once James Gunn took over, as he took away focus from Star-Lord's story, as well as being considered The Scrappy by Gunn.
Stan Lee was originally slated to cameo in the Collector's collection on Knowhere, in which he would have flipped off Groot from within a display case (possibly a reference to the fact that Groot is the only member of the team that Lee had any hand in creating). The scene was cut because Lee was in England at the time and couldn't be reached for filming.
James Gunn reportedly was initially unsure about Chris Pratt portraying Star-Lord.
Thanos almost got cut from the movie, but Gunn decided to keep him for a connection to The Avengers.
James Gunn wanted to have Rom the Space Knight pop up in the movie, but was unable to due so because Marvel does not have the rights to use the character. (It is currently owned by Hasbro after they acquired Parker Brothers.)
An Iron Man cameo was planned but didn't happen - as Robert Downey Jr wasn't sure if he was going to stay with the franchise. He since signed on to do two Avengers sequels.