Bill Anderson (Austin in the original play) is one of the three possible fathers of Sophie Sheridan and a Swedish writer and adventurer. He eventually falls in love with Donna's best friend, Rosie Mulligan, the fun-loving, un-married British author. Bill's original last name was Austin and he was Autralian in the musical, based on ABBA's popularity in that country. When the filmmakers cast Stellan Skarsgård they changed Bill's last name to Anderson and his nationality to Sweden, where ABBA has its roots.
Director Phyllida Lloyd has teased that Bill is possibly Sophie's biological father, and in the musical and uncut film, they shared a song and storyline which many thought it was when they both realized Sophie really really believes that Bill is her father and not Sam or Harry, which means Sophie is much like ABBA who has her roots in Sweden. In the sequel Donna meets Bill after her affair with Harry. The two help a fisherman as he desperately races to his true love, to keep her from getting married. Bill's attempts to seduce Donna fail and she sees right through his womanizing ruse.
After Donna discovers Sam is engaged and breaks up with him, Bill arrives on Kalokairi. Rosie is infatuated with him, but because he is in love with Donna she finds herself jealous. In the present Bill and Rosie broke up because of his unfaithfulness, but when she discovers he can't come to terms with Donna's passing they grieve together and make up. It is also revealed that Bill has a twin brother in the sequel.