Aileen Wuornos, a notoriously uncooperative person, gave director Patty Jenkins access to hundreds of letters she had written and received in order to gain insight into Aileen's life.
Charlize Theron won the academy award for playing Aileen Wuornos on 29 February 2004, Aileen Wuornos's birthday.
Charlize Theron gained 30 pounds for her role in the film.
According to Patty Jenkins, the movie was shot in 28 days, sometimes with as many as four scenes, or nine pages of script, being shot in one single day.
Charlize Theron ad-libbed the memorable line "Fuck you, Leslie!"
Steve Perry from the band Journey was the music consultant.
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.
Kate Winslet was the original choice for the role of Aileen Wuornos.
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.
Kate Hudson was originally offered the role of Selby but turned the role down.
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.
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.
Patty Jenkins wrote the screenplay in only 7 weeks, while the production itself only took 2 months to be set up.
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.
Christina Ricci gained about 10 pounds for her role in the film.
The gun used by Charlize Theron in this movie, was sold at a Beverly Hills pawn shop for $4,500.
The skating rink is called Semoran Skateway, in Casselberry, Florida. Fun World, where Selby and Aileen Wuornos ride the ferris wheel, is in Orlando, Florida on the tourist strip, International Drive.