When Lee is first shown interacting with a john, she shows him a photo of two young children claiming to be her children, and says she's trying to reach them and needed some cash for that. According to the Aileen Wuornos: The Selling of a Serial Killer (1992), the real Aileen Wuornos actually used this method to lure her johns into giving her money in exchange for sexual favors because her physical appearance was starting to degrade quickly on account of her wild lifestyle. She showed photos of her adoptive sister's children claiming to be her own children, which turns out to by her aunt's children (Aileen's cousins, for that matter), since Aileen was legally adopted by her own grandparents after her biological mother left her when she was still an infant.
At the beginning of the film, Lee tells Selby that she has a "pressure cleaning" business. The real Aileen Wuornos mentions in Aileen Wuornos: The Selling of a Serial Killer (1992) that she bought an expensive pressure cleaning kit for her former lover. Soon after, not only the girl left Aileen, but also took the expensive kit with her.
Patty Jenkins has said that she knew Charlize Theron was perfect for the role of Aileen Wuornos when she awoke to a scene of The Devil's Advocate (1997) on television. Jenkins at the time was writing her film Monster. She said that an actress that would allow a closeup of her nose running, as Theron did, would be open enough to pull off the challenging role of Wuornos.
The biker bar scenes were filmed at "the Last Resort" - a bar frequented by the real life Aileen Wuornos, and the site where she was actually arrested. The bar owner (who capitalized on Wuornos' infamy by hanging a sign out in front of the bar advertising "cool beer and Killer Women") makes a cameo as the bartender who threatens to cut off Wuornos for being over her tab limit.
Aileen Wuornos did use a storage facility, like the one seen after she leaves Selby's home the morning after they first met. Although it's shown only before the killing spree in the movie, she actually kept some of her victims' belongings in that storage, while selling others to pawn shops, which eventually led to her thumbprint identification and subsequent arrest.
Scott Wilson turned down the small role of Horton a number of times before finally agreeing to play it. He said he knew that in order to play the part correctly it would require him to go to a place emotionally that he was not sure he was prepared to go as an actor.