"He describes some sense of using the films to get himself in the proper mood to go out and cruise for victims," Dietz said. "And sometimes he would bring the victim home and watch a portion of the film with them."
Dahmer, who confessed to killing 17 people and eating some of their body parts, showed a specific interest in two characters: the creepy Emperor from 1983's Return of the Jedi and the eerie Satan character in the 1990 movie The Exorcist III.
"What these characters have in common is that they are evil and corrupt and powerful, and both have the ability to use special powers to control others," Dietz testified. "He's consistently told me that making others suffer was not his desire sexually, but that he did identify with the power of these characters."
Dahmer, who watched the movies repeatedly, was so enamored with the two figures that he even tried to look like them. Both had yellow eyes, and Dahmer would sometimes wear yellow contact lenses to clubs when he went to scope out potential victims "so he could have more of that look," according to Dietz. "He wanted to be more like those people."
Rader, whose moniker of BTK stood for "Bind, Torture, Kill," was known to conduct surveillance of his potential victims, she says. "It was all part of the routine, because he thought of himself as a spy, and thought of them as the target," Ramsland tells A&E.
Other spy-game tactics, she says, included shopping beforehand to supply his so-called "murder kits," buying different kinds of gloves to see which he liked best and stocking up on knives and ropes. He would also cut the telephone wires to the victim's homes.
In addition, he had "killing clothing," she says, costumes that he would wear to enhance his spy fantasy. Among the characters he would play were a James Bond type wearing a tweed jacket and toting a briefcase, and a telephone repairman wearing a hard hat and coveralls.
All the prep, she says, "is part of setting the mood," Ramsland says.
He had other routines as well. Rader, who strangled some of his victims, got in the habit of squeezing a ball to strengthen his hands to make sure he could effectively murder people. "He realized he wasn't going to be able to do this again unless he exercised his hands," Ramsland says. "He would prepare himself."
As for Bundy, who murdered at least 30 people from 1974 to 1978, he was in the habit of drinking alcohol before some of his killings both as a way to reduce his inhibitions and psych himself up for what was to come, Ramsland says. "It would help him to feel more prepared to do it," she says.
Watch: Naomi Ekperigin talks about the life of serial killer Dennis Rader, aka the BTK Killer, and how the police eventually caught him.
At first, he drank "to do what he was doing," but as his reign of terror went on, he no longer needed the liquor as his liquid courage; he simply liked imbibing, she says.
Christine Sarteschi, an associate professor of criminology at Chatham University in Pittsburgh whose scholarly text Mass and Serial Murder in America includes a case study of Bundy, says it's hard to know exactly how much of a role drinking played in Bundy's murderous activities, but it likely doesn't qualify as the strict definition of a ritual.
"A ritual is a series of prescribed steps that are followed to completion and have no pragmatic value," she says. "They are often associated with religious activities, but not always."
But she agrees that if Bundy was drinking regularly to lower his inhibitions, that mindset would have made it easier for him to set out on the prowl.
It "might increase his boldness," she says, and have prompted him into "going after people in general, and going after people he might not have otherwise, had he not been intoxicated."
