When Jack's mother comes to visit Mary, you can see in the background what appears to be a little boy standing in a doorway. There is a rumor that this is the ghost of a little boy who died in the apartment in which the film was shot. This rumour is false, as the interiors were all shot on a sound stage in a movie studio. The "ghost" is actually a cardboard cut-out of Jack wearing a tuxedo. This prop appears later in the film, when Mary's mother comes to collect her.
Director Leonard Nimoy looked at videotape footage of about 200 hundred sets of twin girls for the part of Mary, before deciding on Lisa and Michelle Blair for the part. Four sets of twins were seen in person before the final choice was made.
During the two week rehearsal period before filming began, then-bachelors Tom Selleck and Steve Guttenberg took the long-married Ted Danson out on the town with them, visiting a few Toronto night clubs, bars and restaurants, to rekindle the feeling of bachelorhood they had to project in the film. The three caused a lot of excitement among the public whenever they went out to party.
Toward the end of filming, the babies that play Mary, were less entranced by the actors around her, and became enticed by the microphone. She would start to follow it with her gaze, and not look at the actors. Leonard Nimoy and other crew members had to start hiding the microphone and disguising it from the baby's view.
Director Leonard Nimoy had some arguments and differences on the set with the lead actors. According to Steve Guttenberg, both Tom Selleck and Ted Danson were used to being the bosses on their own television shows, but soon learned that the real boss in charge was Nimoy. Guttenberg said, "Leonard was a good director, and being an actor, he knew the problems actors face - although sometimes I think he forgot."
The film was pitched to TriStar Pictures, but TriStar passed on it. Then the film was pitched to Universal Studios. But Universal president Sid Sheinberg felt the film was too soft for them. Then they took the project to The Walt Disney Company who took a liking to it.
When it was announced that one hundred dress extras would be hired for a party scene taking place in Tom Selleck's on-screen bachelor pad, five thousand women showed up, thinking they would at least get to see Selleck in person, even if they didn't get the job. The 4900 who were not chosen, did not get to see Selleck or any other film stars, as they were all working on another set. The hundred extras got to party with Selleck for four days. The party scene took a scheduled four days to shoot.
The party scene at the beginning of the movie was going to start differently. One of the extras was going to be on the phone in the lobby, hang up, and then be followed into the party. The extra was unable to get the shot right, and so that entry was scrapped.
Actress Margaret Colin is a frequent love interest for Tom Selleck's characters. She plays his non-exclusive girlfriend in this movie. She's also a TV love interest of Thomas Magnum on Magnum, P.I., and Frank Reagan on Blue Bloods.
The "greatest clutch shot in basketball history" that Peter's guests are watching during the party is from Game 3 of the 1970 NBA Finals. Jerry West of the Los Angeles Lakers sank a 65-foot desperation heave with two seconds left to tie the game, but the New York Knicks won the game in overtime and eventually the series to win the NBA championship. Largely unknown is the fact that, after the Knicks went up by two before West's shot, 'Wilt Chamberlain' never was completely out of bounds after he took the ball to inbound it to West. Chamberlain was frustrated and just casually flipped the ball to West and was heading toward the locker room when West hit the shot.
In the original screenplay, Jack was an airline pilot. Paul Millner had a larger role. The overall film was originally intended for adults as there was much more cursing, a big animosity between Peter and Michael. Also when Peter returns with the formula, originally Michael then went out to Safeway to get the diapers, which Peter forgot.