The Guardians of the Galaxy franchise is known for a lot of things, but the music may be its defining characteristic. Now, with Guardians of the Galaxy Vol. 2 available on home release, it looks like the Awesome Mixes are even more foundational to the stories than we ever realized. In fact, on the Vol. 2 Blu-ray's commentary track, James Gunn laid out how he stockpiles possible Guardians songs, then writes the ones that he eventually uses into his treatments before he even writes the scripts. Per Gunn,
The truth is I have a list of about 500 songs that I think of as "Meredith Quill songs." Songs she would really love. And in that list I keep that on files on my computer and as I'm writing the screenplay, actually as I'm writing the treatment before the screenplay, I'll go through it and find songs that fit a moment or a place that needs a song and I'll search out a song that will fit the moment.
The process of chipping away at a list of 500 songs must be incredibly daunting, but it definitely means that there's no shortage of music to accompany a dramatic (or hilarious) Guardians scene. It's important to remember that the soundtracks for the first two Guardians of the Galaxy movies cannot include any music made after the death of Meredith Quill. James Gunn specifically looks for songs that Star-Lord's mother would actually listen to, and he then tries to find ways to weave them into the story beats of a given film in a way that makes sense. Whether it's "Hooked on a Feeling" for a triumphant escape, or "Father and Son" for Yondu's funeral; it all serves a purpose.
From there, the way James Gunn pitches the songs to Marvel is delightfully old-school. The director continued and explained that he actually burns Marvel a mix (not unlike the one Meredith gives Peter) to accompany the treatment that he turns in to the studio. Gunn explained:
Sometimes I'll go outside of that list of 500 songs and go a little bit deeper and find something that is more appropriate, which I'll do at times, but I write them all into the treatment. Then when I give everyone that original story I'll give that story to Marvel and I'll give them a compact disc, and old-school compact disc with the songs on it so that they can listen to all of the music along with the treatment and they know exactly what we're getting into.
James Gunn has been pretty lucky in regards to his music choices for the Guardians of the Galaxy movies so far. Elsewhere in his commentary, he noted that between his two entries in the Marvel Cinematic Universe, Marvel Studios has yet to deny a request to use a song in a scene. Considering the massive success that these movies have achieved, it only seems fair to assume that Marvel may continue granting Gunn's song requests for the foreseeable future.
Of course, it's worth mentioning that this could change when Guardians of the Galaxy Vol. 3 rolls around. Now that Star-Lord has a Zune instead of his mother's Awesome Mixes, the song selection will be geared more towards Yondu than Meredith Quill in terms of inspiration. That said, it doesn't look like James Gunn's technique of weaving his songs into the early incarnations of the story will change anytime soon.