Reitman contacted Dawson, and, after reading the film's screenplay, she agreed for her songs to be used in the film, sending him a packet of CDs containing about 120 songs. The songs were almost entirely self-published by Dawson, who says she wrote nothing specifically for Juno and that all the songs had been performed and recorded before she was contacted to work on the film. Reitman asked her to additionally re-record instrumentals, which included humming over the lyrics of some of her songs. He also contacted composer Mateo Messina, with whom he had previously worked on Thank You for Smoking, to compose the film's incidental score. He gave Messina a collection of Dawson's songs and asked him to create "the sound of the film" through an instrumental score that replicated the recording quality, tone, feel and innocence of her music. Messina decided to implement an "acoustic guitar feel that was jangled and was really loose, like Juno." Experimenting with different guitars, he ended up using "Stella," a second-hand guitar belonging to guitarist Billy Katz that he described as "kind of tinny, not perfectly in tune, but [it] has all kinds of character." Katz was hired to perform acoustic and classical guitar for the movie's score, using "Stella" extensively throughout.
