Over the course of his career, former FBI agent and behavioral analyst John E. Douglas has interviewed criminals ranging from repeated hijacker Garrett Trapnell and cult leader Charles Manson to serial killers Edmund Kemper (a.k.a. the Co-Ed Killer) and Dennis Rader (a.k.a. B.T.K.). In his new book, The Killer Across the Table, Douglas takes readers into the room as he interviews four very different offenders.

In these conversations, “I'm trying to gain [their] trust [to get] information that I'll be able to apply to current cases,” Douglas tells Mental Floss. Here, he outlines how he prepares for an interview with a killer to figure out what makes them tick.

1. Never go into an interview cold.

“Preparation is the number one factor for a successful interview” of this kind, Douglas says. “Before I go in to do an interview, [I] go back into the files and fully look at the case that got him or her incarcerated to begin with. Which means looking at the police reports, the preliminary protocol that the medical examiner did regarding the autopsy, autopsy photographs, and then looking in the corrections reports as well. You want to be totally armed with the case when you go in.”

2. Memorize everything—don’t use notes or a tape recorder.

Early on in his interviews with killers, Douglas used a tape recorder, which he now says was a mistake. “You're dealing with very paranoid individuals. They don't trust you, they don't trust the [corrections] system,” he says. “If my head is down, [they’ll ask], ‘What, are you taping this? Why are you writing these notes down?’” Memorizing the case is key—when he goes in, he won’t have notes or a tape recorder: “It's going to be key [for] me to maintain some eye contact with them.”

3. Make sure the environment is right.

The key in these interviews, Douglas says, is to make the environment feel open so that the killer feels comfortable and like he’s in control. “When you go into a prison, sometimes you're forced to deal with what you've got,” he says. “But if I have time, I try to [make arrangements] depending on the personality.”

Douglas prefers to conduct his interviews at night, relying only on low table lights to create a soothing, stress-free atmosphere. Douglas will even think about seating arrangements. “If I'm dealing with a real paranoid type of individual, I need to put this person near a window—if there's a window—so that he can look out the window and psychologically escape, or I may have him face a door,” he says. Both Charles Manson and Richard Speck chose to sit on the backs of their chairs so they could look down on him. Douglas’s attitude is: “You hate me. I know you hate me, but go ahead and do it. I'm just trying to get a little bit of information now.”

4. Don’t rely on what a killer tells you.

Douglas never takes a killer’s word for anything, which is why memorizing the case is so important. Typically, he knows the answers to the questions he’s asking, and it allows him to call out the offender if he or she lies. “If you don't look deeply into the material, you don't know who in the heck you're talking to,” Douglas says. “You're talking to somebody who's pulling the wool over your eyes … If [an interviewer relies] on self-reporting, they're going to be filled with a lot of lies coming from the person they're interviewing.”

5. Know that this is not an interrogation.

Once he knows who’s committed a crime, Douglas says, his main goal is to find out what motivated them. The best way to get that out of them is to ask his questions “in a very relaxed kind of a format, making the subject—even if it's a guy like Manson or some of the worst killers you'd ever want to meet—feel real comfortable and feel at the same time that they are controlling me during the interview.”

What Douglas ultimately tries to do is have a conversation with the offender. “That means if they're asking me a lot of questions about myself, about maybe my family, my job, and I'm pretty honest with them,” he says. “They will trust me and open up to me as long as they know that I know the case, backwards and forwards. If they start fudging on the case trying to send me down the wrong path, I will confront them, but not in mean [way]. I'll laugh and say, ‘Look, come on. I know what you did. What are you doing here?’ That’s how you gain their trust.”

6. Be mindful of your body language—and the actual language you’re using.

When he’s in an interview, Douglas isn’t sitting there with his arms crossed, looking uncomfortable. “The body language should be just relaxed, not a defensive kind of posture,” he says. “[It should be] very comfortable—like on a date kind of thing.”

Douglas also avoids words like killing, murder, and rape, and, as awful as it might sound, avoids placing the blame on the killer. “I'm trying to get him to talk so we're going to project the blame," Douglas says. "[Some killers] use this projection, never accepting responsibility, not admitting that it was free will, that they had the ability to make choices and they made the wrong choices in their lives, even though they may have come from a very, very bad background.”

This kind of approach is what helped Douglas gain insights from Ed Kemper. When Douglas asked how Kemper—who was 6 feet, 9 inches tall and 300 pounds—would get young women in his car, Kemper revealed that he would pull up next to them and look at his watch, which would give them the impression that he had somewhere to be. “I’ll go with this guy. He’s got an appointment, nothing’s going to happen to me,” Douglas says. “Just a little thing like that was real helpful to me.”

7. Play it cool, no matter what happens.

Being confrontational is no way to get a killer to open up. “In an interview, whether it's a serial killer or any type of violent offender, I'll never challenge them or be negative toward them,” Douglas says. “I'll never do anything like that. If I feel that they're not being truthful, I'll bring it to their attention. But I’m on a fact-finding mission. There are several shows on television right now where celebrity types are going into prisons doing interviews. They get in the guy's face and they call him a liar. [So] the guy, what does he want to do? ‘I want to go back to my cell. Screw you. I'm out of here.’ And you can't hold him there—he's got to go back. So, you never do anything like that.”

8. Don’t be afraid to feign empathy.

Sometimes getting what you need out of an offender means fudging the truth. Sometimes Douglas will tell the killer that he’s earning points with the warden by doing the interview. “There's still always this glimmer of hope that they'll get out of prison one day, even if they're in there for multiple murders,” he says. “The warden doesn't give a damn about him, but I'm just telling them this to try to get him to speak up.”

Sometimes Douglas will play to his subject's pride and narcissism. “They want to be the big daddy,” he says. "'But I'm the main guy, right? You're doing this research and you guys got the real McCoy here. I'm the best and the worst of the worst.'" And sometimes, he feigns empathy—all with the goal of finding out information that will help prevent and solve other crimes.

"Let the person feel they are in control of the interview,” Douglas says. "Be open with yourself. Give them information about yourself to this person and it should go well."