The shot of Will reflected in the mirror when he is depressed was actually taken when Hugh Grant was taking a break. The directors noticed him doing this and from behind he looked depressed, so they shot it without him knowing.
In the novel, Will introduces Marcus to, who bonds with Ellie over, the band Nirvana. The book is set in 1993-94, and the story climaxes with the kids learning about, and being affected by, lead singer Kurt Cobain's suicide. The title of the story is an allusion to the Nirvana song "About A Girl".
Brad Pitt turned down the lead role of Will Freeman on the grounds that it was implausible that someone so attractive would need to pretend to be a single father to meet women. He nevertheless appears in the film (sort of); he is on the cover of an issue of "Esquire" that Will is reading in his flat.
The quote "No man is an island" which Will jokingly attributes to Jon Bon Jovi, actually was written by John Donne in 1624, in his work "Meditation XVII".
At the beginning, Marcus says he would be able to take care of his mum (Toni Collette) if he were Haley Joel Osment. Toni Collette played Haley Joel Osment's mother in Sixième Sens (1999).