Jenna Fischer, who was married to Gunn at the time, was not originally part of the cast. The actor in her role begged to be released from his contract because he had an offer to shoot a pilot, so Gunn let him go rather than have a performer on set who didn't want to be there. Gunn swapped the character's gender and gave her a few more lines. By the time the movie was released Fischer had become immensely popular from The Office, so she was the one who went on The Tonight Show the night before the film opened.
Rooker dislocated his shoulder while filming the scene where he attacks Starla and grows a floppy arm. It was a long shooting day with multiple setups, and he didn't tell anyone until the shot was in the can.
In the opening scene as they pan down the street, you can see "R.J. McCready's Funeral Home". R.J. McCready is the name of the mayor, and was the name of Kurt Russell's character in La cosa (El enigma de otro mundo) (1982). Russell ended up working with James Gunn 11 years later on Guardianes de la Galaxia Vol. 2 (2017).
The alien slug swimming towards Kylie in the foamy bathtub is also a nod to A Nightmare on Elm Street (1984) when Freddy's bladed glove emerges between the legs of a sleeping Nancy while she's taking a bath.