To improve the relationship between the leads, director Nick Cassavetes staged an intervention by bringing them into a room where they could air all the grievances they had with each other and work something out. They soon patched over their differences, enough to become a real-life couple for some time.
Rachel McAdams auditioned for the role during another movie's premiere. She was given the script just a day in advance and beat nine other actresses for the role. The scenes used during her audition included the one of Allie's and Noah's argument at the end after Allie and her mother return from their morning drive.
The film rights to the novel were acquired by New Line Cinema in 1996, represented by producer Mark Johnson. Jeremy Leven was hired to write the script, which caught the attention of Steven Spielberg in 1998, who wished to film it with Tom Cruise as Noah Calhoun. Spielberg's commitment to other projects led to Jim Sheridan becoming attached to direct the following year. Filming was to start in 1999 but was pushed back over rewrites. Sheridian eventually backed out by October 2000 to work on In America (2002).
Rachel McAdams and Ryan Gosling were both born in London, Ontario, Canada. Walt Whitman, quoted in the classroom scene ("Do I contradict myself?") and other parts of the movie, had a long friendship with 19th century London, Ontario, psychiatrist Dr. Maurice Bucke, which was depicted in the film Beautiful Dreamer (2006).
The trivia items below may give away important plot points.
The "'after 7 years' Allie and Noah" scenes were shot first, and then the crew/cast went on a Christmas break. Ryan Gosling had to lose the beard and 20 lbs he'd had as the older Noah and come back to play the young Noah. Rachel McAdams and Ryan Gosling were rather shy and unsure around each other in the "after 7 years" scenes but were more comfortable together in the young Allie and Noah scenes.
The pictures of an older Allie and Noah through the years are actually the personal photos of Gena Rowlands of her and her husband John Cassavetes with James Garner's face photo shopped in over John's face.