He spent much of the period from 2005 to 2007 working on a musical adaptation of The Princess Bride with original screenwriter William Goldman. As of January 2007, Guettel had written the music for ten songs for the project. An orchestral suite from the score was performed at the Hollywood Bowl in November 2006, and Lincoln Center conducted a workshop of Bride in January 2007. The project was abandoned when Goldman reportedly demanded 75 percent of the author's share, even though Guettel was writing both the music and the lyrics.
In a 2003 profile in The New York Times, Guettel discussed, among other things, his history with addiction. Published during the Seattle tryout of The Light in the Piazza, he spoke with regret about an earlier time in his life when these struggles hindered his creative productivity.
While Guettel is best known for his work in the theatre, he is lesser-known for his work in environmental conservation. His lifelong concern for the environment was sparked at age seven, when he read The Lorax, by Dr. Seuss. Today, his conservation efforts are primarily carried out on behalf of family dairy farms in New York State and Vermont, where he has a residence. His interest in Dr. Seuss carried into his professional life: at age 19, he composed and orchestrated a one-act opera based on The Butter Battle Book, which was never produced due to rights negotiations.
When Guettel took up music composition in his mid-teens, he was encouraged by his family. His mother said that she offered him advice for around a year, "After that, he was so far beyond anything I could ever have dreamed of, I just backed off." Richard Rodgers, who died when Guettel was 15, overheard an early composition, said he liked it and asked him to play it louder. Guettel has qualified the compliment, noting that "He was literally on his deathbed on the other side of the living-room wall." In his high school and collegiate years and into his early twenties, Guettel worked as a rock and jazz musician, singing and playing bass, before realizing "that writing for character and telling stories through music was something that I really loved to do, and that allowed me to express love."