Sigourney Weaver originally declined to do a fourth Alien film. When asked why she changed her mind, she replied, "They basically drove a dump truck full of money to my house."
Jean-Pierre Jeunet wanted to have a scene where a mosquito bites Ripley, then vanishes into smoke because of her acid blood. Eventually, he dropped the idea after the SFX team told him how much it would cost.
Winona Ryder agreed to do this film even before reading the script. She stated that she "didn't care if she died in the first scene", she'd do it. Ryder claimed that then she could boast about being in an "Alien" movie to her younger brothers.
(at around 24 minutes) Sigourney Weaver successfully made the behind-the-back half-court basketball shot after three weeks of practice, tutored by a basketball coach. Her conversion rate during this time was 1 of over 6 shots, but the distance was much shorter than it would be in the actual scene. When the day came to shoot the scene, director Jean-Pierre Jeunet wanted to have the ball dropped in from above, rather than wait for Weaver to sink the shot herself, which "would probably take about 200 takes." Weaver insisted she could make the shot and was allowed to try. Though it is commonly said that she sank the basket on her first attempt, it actually took her many takes to complete the stunt. Ultimately, Jeunet gave her one last try to sink the basket before they would give up and use CGI or a second ball. The very next take, Weaver successfully managed the trick. Ron Perlman broke character and began smiling when he saw it, and people on the set started cheering. The editors looked at the shot and decided there was "enough room to get the scissors in." Weaver was excited about making the shot, but Jeunet was concerned audiences would believe it to be faked due to the ball leaving the frame. Upon Weaver's insistence, he kept the shot as it was. Weaver described the miracle shot as "one of the best moments in her life," after her wedding day and the birth of her daughter.
The full-size Newborn animatronic puppet was originally filmed with genitals that were a mix between male and female genitals. They had to be digitally removed on studio orders.