Contrary to popular belief, the identity of Sophie's biological father is definitively known by the writers - it's Bill Anderson, played by Stellan Skarsgård.
The character of Pepper, one of the young male servers, was originally written as a much larger role but was reduced due to time constraints.
Meryl Streep insisted on doing all of her own singing as Donna, despite having limited experience as a musical performer prior to the film.
Lisa, one of Donna's former bandmates in Donna and the Dynamos, was supposed to have a more prominent role but was heavily reduced in editing.
The characters of Harry and Bill were initially conceived as a gay couple, but this subplot was removed to maintain a family-friendly tone.
Stellan Skarsgård had to relearn how to play the guitar left-handed for his portrayal of the musically-inclined Bill Anderson.
The character of Eddie, Pepper's best friend, was originally written as a much older man but was aged down to a younger, scrappier version.
Julie Walters based her portrayal of Rosie in part on her own experiences as a member of an all-female singing group in her youth.
Amanda Seyfried's casting as Sophie was almost derailed when the producers initially worried she was too old to play a 20-year-old.
The Mamma Mia!
franchise is notable for its diverse representation, including same-sex relationships and a transgender character in the sequel.