In one scene the dog starts mimicking Jack Nicholson by stepping over the cracks in the sidewalk. The filmmakers accomplished this by placing little obstacles on the cracks so that the dog had to step over them. They then removed the obstacles digitally in post-production.
At the 1998 Oscar ceremony, when his name was announced as the winner of the Best Actor award, Jack Nicholson ascended to the podium by carefully stepping over the cracks between the tiles on the stage, in imitation of his character, Melvin Udall. Nicholson dedicated his Oscar to his Des hommes d'honneur (1992) co-star J.T. Walsh who had died shortly before the Academy Awards in 1998.
Helen Hunt took a two-year break from movies after this film. She'd been working almost non-stop since she was nine years old.
Jack Nicholson wasn't supposed to kiss Helen Hunt, but James L. Brooks yelled to Nicholson, "Kiss her, man!"