During filming the cast became obsessed with playing the murder game (a game in which one person who is the murderer has to find a subtle way to kill other players simply by looking at them without giving themselves away as the murder). This is very apparent in the dinner scene where everyone is sitting around the table. If you watch everyone's eye movements it's obvious they are playing the game.
In the scene where Gil and Karen are going to Kevin's school to speak to the school principal, the school is Gerald Paris Elementary School, named as an homage to Jerry Paris, who directed 42 episodes of Happy Days (1974).
In a 2016 interview, Helen Mirren said she had never regretted not having children, except briefly when watching this film: "It was about the whole story of being a parent and how it never stops, even when you're a grandparent. I realised I would never experience that, and for about 20 minutes, I sobbed for the loss of that and the fact that I never experienced it."
Martha Plimpton was bald at the beginning of filming because she had just finished Silence Like Glass (1989) in which she played a cancer patient. She wore a wig throughout filming and a hairpiece when Julie has a mohawk.
When Grandma (Helen Shaw) inhales the helium balloon and says that when she was born, Grover Cleveland was President. Shaw was born on July 25, 1897, just four months and three weeks after the end of Cleveland's second term.
According to the interview in the DVD extras with composer Randy Newman, the soundtrack song "I Love to See You Smile", which is perhaps his most beloved, was written with Mary Steenburgen's smile in mind.
The movie was filmed in central and northern Florida. The shopping center where the photo booth is standing is in College Park, Florida. The photo booth was constructed for the movie. The Showbiz Pizza is located in Altamonte Springs, Florida and is still there, except it is now (like all Showbiz Pizzas) Chuck-E-Cheese. Kevin's birthday party was filmed at The Mystery Fun House in Orlando, Florida.
In the opening scene at the ballpark, the base runner has a decided limp because he pulled his hamstring on one of the first dozen takes of the shot. The shot in the film is one they settled on after approximately 20 takes, due to the fact that the hitter was unable to hit the ball into center field. The hitter was a college player, not a professional as originally dictated to the casting department.
As originally written, Karen would smoke a joint of marijuana every night. Mary Steenburgen argued that this was out of character for a woman strong enough to confront her family's problems head on. The filmmakers agreed and changed the script.
In the scene where Steve Martin and Mary Steenburgen are fighting over the fact that she is pregnant, Steve says "...Let's just have a dozen and act like they are donuts." In the movie Cheaper by the Dozen (2003) he plays the father of a dozen children.