Director James Gunn promises to share the full mixtape with fans soon

Guardians of the Galaxy features a tree-like creature named Groot, a talking raccoon and an electronic device just as alien to children of the digital age: a Sony Walkman containing a mixtape packed with '70s pop hits like 10cc's "I'm Not in Love," Rupert Holmes' "Escape (The Pina Colada Song), The Runaways' "Cherry Bomb," and David Bowie's "Moonage Daydream."

The archaic device belongs to Guardians lead, Peter Quill, aka Star-Lord, played by Parks & Recreation actor Chris Pratt. "It wasn't in the comics, and it wasn't in the original script," explains James Gunn, 43, co-writer and director of the hotly anticipated Marvel Comics adaptation.

Gunn added the plot device to establish Quill's attachment to earth and an emotional connection to his late mother (who gave him the Walkman and the tunes). Gunn simply chose songs he believed Quill's mother would have liked, but he says, "There's also a sort of 'spacey-ness' about the tracks that reminds me of the [Marvel] comic-book writer and artist Jim Starlin."

Gunn chose songs for the movie by scouring Billboard's singles charts "for every year in the '70s," compiling a master list of 500 songs. He ultimately trimmed it down to 120 songs and then a 90-minute mixtape. When Pratt was hired, Gunn says, "he asked if he could have the mixtape” to prepare for the role. Gunn says he will likely release the full list to fans in the future.

At the top of his list: Jackson 5's "I Want You Back," which Gunn says "is the greatest pop song ever written." Gunn also enjoys Redbone's "Come and Get Your Love." "I still love that song after probably listening to it for 10 hours straight in the editing room and then also on set. I think that's a testament to how great these songs are -- that I still like them," he says.

With so many choices, some songs just didn't make the cut. According to Gunn, he most regrets cutting a scene featuring Electric Light Orchestra's "Livin' Thing" from the film. "If there was a house band for Guardians…it would be ELO," he says.

Not only was the soundtrack written directly into the screenplay, Gunn also played them on set alongside Tyler Bates' score, which was also composed before filming. "We really baked the music into the movie," says Gunn. "Having it be part of the initial filmmaking process as opposed to something that you tack on at the end...it really made it a much more holistic experience."

Despite the Walkman's importance, Gunn says he will likely take another approach in the follow up. "To keep up the true tradition of the movie…it may not have the music playing in the same exact way as the first one," he says, also hoping other filmmakers will feel the same for their films. "We did this from the heart, so if someone tries to copy it, I'm not sure it would work."