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.
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" magazine that Will is reading in his flat.
In the novel, Will introduces Marcus to, and 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 a pun on the Nirvana song "About A Girl".
As Will describes web-based research as a "unit of time" in his day, he types the URL supermodelswithseethroughtops.com. This is an active Web site, consisting of a picture of Pope John Paul II in his "Pope-mobile".