Harpo Does Something Funny: By the time Eddie Murphy was attached to the project, the script had been shopped around considerably and rewritten several times. Many of the funniest scenes just resulted from Eddie and the other actors improvising in place of the scripted lines.
Life Imitates Art: During his tirade at the Beverly Palms Hotel, Axel pretends to be writing an article called "Michael Jackson: Sitting on Top of the World" for Rolling Stone magazine. In real life, Playboy ran an article called "Eddie Murphy: Sitting on Top of the World."
Serendipity Writes the Plot: In the script, Axel stuffs potatoes he stole from the hotel kitchen into the tail pipe of Rosewood and Taggart's car. Due to time constraints, no scene from the kitchen could be shot. Because the hotel lobby was already a location for a few scenes, the script was re-written so Axel takes bananas from a buffet in the lobby.
Star-Making Role: Eddie Murphy had already carried SNL through its most disastrous season and proved that he could hold his own against white co-stars in 48 Hrs. and Trading Places. This was the film that proved, once and for all, that he had the star power to headline a blockbuster on his own.
Throw It In!: There's a moment where Maitland has Axel and Jenny cornered and is deciding what to do with them when he stop in his tracks and makes a face. In reality, the actor was reacting to something Eddie Murphy ad-libbed but was removed in editing. They kept in his reaction because it made Maitland seem even more sinister and unhinged.
There are a spectacular number of ad-libs in this movie in general, most of them from Murphy.
Unintentional Period Piece: The first film is very much a product of 1984 from the clothing to the music to the pop culture references.
Amusingly, Stallone had a bunch of ideas about how the movie should go and was editing the script accordingly. When Murphy was brought on, Stallone gracefully backed out, taking the ideas with him to be used in Cobra.
The original finale for the Stallone draft of the script took place at night and ended with a car chase between Victor in a Lamborghini and Axel in a turbo-boosting Pontiac GTO. Victor is ultimately killed when his car smashes into an oncoming train.
In the Stallone draft, Billy Rosewood was originally killed off halfway through.
Originally, two men were supposed to be working in the art gallery scenes. When the director heard Bronson Pinchot's Serge impersonation, however, he thought it was so hysterical he scaled back the other part to give Pinchot more screen time. The second actor shows up only briefly with his shirt collar open too wide, on which Serge comments.