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 this film. She said that an actress that would allow a close-up of her nose running, as Theron did, would be open enough to pull off the challenging role of Wuornos.
Aileen Wuornos used a storage facility, like the one seen after she leaves Selby's house the morning, after they first met. Although it's shown only before the killing spree, she actually kept some of her victims' belongings there. She sold others to pawn shops, which eventually led to her thumbprint identification, and subsequent arrest.
When Lee is first shown interacting with a john, she shows him a photo of two young children, saying they're her children, and she needs some cash to get to them. According to Aileen Wuornos: The Selling of a Serial Killer (1992), the real Aileen Wuornos actually used this method to lure her johns into paying for sexual favors because her physical appearance was starting to degrade quickly due to her wild lifestyle. She showed photos of her adoptive sister's children, claiming they were her own children. Wuornos' grandparents legally adopted her as an infant after her biological mother abandoned her.
Scott Wilson turned down the small role of Horton several times before finally agreeing to play it. He thought to play the part correctly, he would have to go to a place emotionally that he didn't think he was prepared to go as an actor.
The biker bar scenes were filmed at The Last Resort, a bar frequented by the real-life Aileen Wuornos, and the place where she was arrested. The owner, who capitalized on Wuornos' infamy by hanging a sign in front advertising "cool beer and killer women", has a cameo as the bartender who threatens to cut off Wuornos for being over her tab limit.
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. She left Wuornos soon after, and took the expensive kit with her.
When out job hunting, Aileen mentions something about "giving the baby up" when she was thirteen. The real-life Aileen Wuornos did indeed get pregnant at the age of thirteen from hooking in her free time outside of school, and was made to give the child, a son, up for adoption immediately afterwards.