Hollywood city officials hung Christmas decorations on Hollywood Blvd. a few months early so that the scenes shot for this film, particularly the action scenes near the end of the picture, looked like they happened at the end of the year.
An alternate opening and ending were both filmed (and are available on the Lethal Weapon 4 DVD). The alternate opening featured Martin Riggs drinking alone in a bar where he is accosted by a couple of thugs who want his money. Riggs claims all of his is in the bank and tells the thugs "not to fuck with him." The thugs attack him, but Riggs easily subdues them. He is then allowed to take a free bottle of booze from the bar in exchange for never returning. Director Richard Donner felt the movie should open with a brighter look at Riggs and filmed the scene with Riggs awakening in his trailer to replace it. The alternate ending featured Riggs and Murtaugh saying good-bye to one another. Murtaugh tells Riggs he's thinking of retiring, but Riggs tells him not to.
Jackie Swanson did perform the high fall on her own. Trained by legendary stuntman Dar Robinson. Also, the stunt was done using an airbag covered with a life-size painting of the driveway and cars, which, like a foreground miniature, visually blends into the real scene. Thus, the editor is able to hold the shot until just as she makes contact with the airbag, for greater realism.
On E! True Hollywood Story, Gary Busey says he was hired to play Joshua because they were looking for someone big and menacing enough to be a believable foe for Mel Gibson. Busey also credits the film for reviving his failing movie career.
A running gag in the Lethal Weapon film series is the "One, Two, Three" false starts where Riggs and Murtaugh can't decide whether to go "One, Two, Three... then go!" or "One, Two, THREE!" (go ON "Three") While the gag does not exist in this film, there is a "One, Two, Three" false start in this movie. It happens when the uniformed cops are trying to prepare to sing "Silent Night" in a chorus and one of the cops keeps starting too soon.
After the Explosion at Dixie's house, Sgt. Murtaugh calls out on the police radio that there is a "fire and explosion at 111th and Larch". This is one-half block west, the nearest intersection of the actual house used during filming in Lennox, CA (4521 111th St).
Despite popular misconceptions, Roger Murtaugh (Danny Glover) never actually says, "I'm getting too old for this shit" in this movie. He simply says, "I'm too old for this shit." He does however say he's "getting too old for this shit" in the sequels.
There is a scene early in the movie that takes place in Riggs' trailer. In that scene Riggs puts his gun in his mouth for another suicide attempt. Right after he does so the camera cuts to the television. The program shown on the television is the Christmas special "Bugs Bunny's Looney Christmas Tales".
Tom Atkins (Michael Hunsaker) reveals that he worked for an operation called Air America which involved the CIA and dealt with shipping heroin out of Laos. Mel Gibson starred in a movie titled Air America that may have been based on this conversation. However, it is equally possible that Hunsacker (as well as Air America) was referring to the historically factual Air America, an American passenger and cargo airline covertly owned and operated by the Central Intelligence Agency from 1950 to 1976. It supplied and supported covert operations in Southeast Asia during the Vietnam War.