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 A Few Good Men co-star J.T. Walsh who had died shortly before the Academy Awards in 1998.
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.
Betty White, an animal rights activist, was offered the Shirley Knight (Beverly Connelly) role, but turned it down when she learned of how the dog would be mistreated in the movie.
Title of this movie was translated as "Mr. Cat Poop" in Hong Kong. Apparently it came from the name "Melvin" which is pronounced very similarly to the Cantonese colloquial word for cat poop.
An earlier version of the script was called "Old Friends". It was listed as one of the best unproduced screenplays by Movieline magazine and had Kevin Kline and Mike Newell attached at one point to star in and direct it, respectively.
The couple Melvin insults at the restaurant in the beginning of the movie are played by Peter Jacobson and Lisa Edelstein, who would go on to star together in the TV show House M.D..
In _Ironweed (1987)_, made ten years before his film, Jack Nicholson tells Meryl Streep, "By God, Helen, that's as good as it gets," after her character sings the song, "He's Me Pal," in his honor.
Courtney Love turned down the role of Carol as she was recording the 'Celebrity Skin' album with her band Hole. She also felt she was too young for the part at the time.
A bit of humor was added right after the statement "the animals used in this film were in no way mistreated" with "the actors used in this film were in no way mistreated."
Woody Harrelson read for the part of Simon. According to Harrelson, he "went too femme on it". The audition took place at Nicholson's home and Harrelson admits that he was nervous meeting him.
At the end of this film, when the credits begin to roll, the song "Always Look on the Bright Side of Life" performed for this film by Art Garfunkel, begins to play. Jack Nicholson and Art Garfunkel starred together in the film, Carnal Knowledge.