Brennan conceived the idea for Glee based on his own experience as a member of the Prospect High School show choir. He explained: "I find it interesting that there is something in everybody, a longing for something transcendent, particularly in a place like Mt. Prospect, a place that’s very suburban and normal and plain. Even in places like that, there’s this desire to shine. That’s fascinating and very funny to me, especially when people try to accomplish this through show choir – which, to me, is inherently a little ridiculous." In 2005, a choir director at Prospect High was accused of inappropriate behavior with a student and was later convicted of aggravated criminal sexual abuse. Brennan was inspired by these events, and in August 2005, purchased a copy of "Screenwriting for Dummies" and wrote the first draft of Glee, initially envisioned as a film rather than a television series. He completed the script in 2005, but could not generate interest in the project for several years.
Mike Novick, a television producer and a friend of Brennan's from Los Angeles, was a member of the same gym as writer and producer Ryan Murphy, and gave him a copy of Brennan's script. Murphy had been in a show choir while at college, and as such felt he could relate to the script. Murphy and his Nip/Tuck colleague Brad Falchuk suggested that Glee be adapted to a television show format. The script was entirely re-written, and was picked up by Fox within 15 hours of being received. Murphy and Falchuk became the show's executive producers and showrunners, while Brennan is a co-executive producer and Novick is a producer. Brennan, Falchuk and Murphy wrote all of the show's episodes in the first two seasons, and were nominated for two 2010 Writers Guild of America Awards for Glee, with nominations in the "Comedy Series" and "New Series" categories.