"He's always on the hunt, looking for potential victims," said Douglas. "They only act out when they feel that things are safe. They're not sloppy or careless. They can't control themselves, and in the break between these crimes, they fuel their fantasies by taking mementos."
In Harvey's case, it was Susie's charm bracelet. "He almost gets caught when the detective comes looking around and sees the doll houses," he said. "Harvey grabs the bracelet and hides it. He discards it later, but keeps one single charm."
"Their wish is to spend hours, days, months, a lifetime with the victim," said Douglas.
"But it never works out that way," he said. "In the fantasy everything works perfectly. Sometimes they think the victim will be favorable to their advances. But the child starts crying and wants to go home and they lose that control."
Douglas, 64, helped develop the FBI's criminal profiling program in the 1970s and then got actively involved in hunting down some of the nation's biggest predators -- Wayne Williams, the Atlanta child murderer; Gary Ridgway, the Green River Killer; and David Carpenter, San Francisco's Trailside Killer.
In face-to-face interviews with Charles Manson, David Berkowitz (Son of Sam) and Richard Speck, he gleaned methods to predict the behavior of a serial killer -- from the escalation of fantasy to violence and even victim selection.
"When I interview I try to visualize what it's like when the victim looks at them," he told ABCNews.com. "They can turn on a dime -- go from nice guy to bad guy within the snap of a finger."
"The Lovely Bones" takes place in 1973, but today, with more sophisticated investigation techniques and tools, that kind of case would be "very solvable," according to Douglas, a self-described "creative right brain kind of guy."
In analyzing the fictional Harvey, Douglas saw a "level of sophistication" in the man who was able to conceive and create a cell where he entrapped his victims. That trait is also evident in the dismembering the bodies.
"If I didn't know who was responsible for this homicide, I would know he had done others in the past," said Douglas. "We're not looking for a teenager, but an older, criminally sophisticated offender who lives in the area, who feels comfortable. The focus should be right there, in the neighborhood."
During police questioning, the killer is often "over-cooperative," with clothing that is "buttoned up and not sloppy" that shows a rigidity of personality, according to Douglas. Usually the house is orderly.
Killers often maintain newspaper clips and scrapbooks of their victims and crimes. The Kansas "blind, torture and kill" (BTK) strangler Dennis Lynn Rader, apprehended in 2005, built a torture chamber for his victims and made drawings of their demise.
Douglas's career with the FBI began when he was only 25. In 1977, he was assigned to what was then FBI's training headquarters -- now the behavioral science unit -- in Quantico, Va.
"Without a doubt, John Douglas had this intuitive ability, better than anyone I have ever met," said Ken Lanning, a retired agent who for 15 years worked across the hall in the windowless basement offices at Quantico.
"He was laid back and easy going, very good at reading people," Lanning said.