criminalprofiler:

Dr. Robert Keppel interviewed Bundy extensively and wrote about his encounter with the killer in his book The Riverman: Ted Bundy and I Hunt the Green River Killer.

Keppel remembers the advice given to him by two mentors about interviewing these types of killers:
“I remember some guidelines they went over with me. first, question the interviewee at his level. Use the same words that he uses and the same sentence structure. This would make Bundy feel comfortable. Don’t ask questions over his head intellectually and, at the same time, don’t ask questions below his intellectual level or he will never cover what you want.”

In October 1984 Bundy sent a letter to the Green River Task Force. He believed that he could provide some insight into the Green River Killer’s behavior. The letter was handed over to Keppel, who replied, and a month later arranged a trip to go and interview Bundy. To avoid giving Bundy a feeling of importance, Keppel acted as though his trip to visit Bundy was a side trip:

“I made it appear that my trip to Florida was for other reasons, and I had just stopped by because it was no trouble and I was so close by, anyway.”

Keppel developed ways to try and determine if Bundy, a professional liar, was trying to deceive him:

“One strategy was to ask Bundy a question that was blatantly false and determine his style, looks, vocabulary, and sentence structure when he defended the truth. He’d lied so much over the years that he looked highly comfortable when doing it.” Bundy would sometimes answer in Keppel believed was an uncharacteristically vague style, and Keppel realized why. “He would talk a lot about nothing when things approached the truth about him. I believe he wanted to confess so bad that he would be void of detail when his answers were close to what he would have done. This was a signal to me that I was getting dangerously close to his self-admission. This was a signal that he was answering things about himself. Go any closer and he might clam up.”

Another strategy was to ask questions that would seem like they were about the Green River Killer but could also be about Bundy’s methods and motives.

“The questions we asked, such as why would a killer murder two victims on the same day, were the type of questions we might ask about the Green River Killer. We hoped we might get Bundy to use the first-person when answering.”

Keppel believes that when he returned to talk to Bundy, four years later, Bundy wanted to confess, but his advisers were against it as it could hurt his appeals. Keppel describes how Bundy began telling him how to interview serial killers. Bundy suggested that it was best for the interview to leave aside all judgment.

Keppel went back to visit again before Bundy was executed. Bundy was ready, and told Keppel that he often beheaded his victims, he revealed the names of some victims to help close some cases, and confirmed other names on Keppel’s list but resisted admitting to others. Keppel believes that Bundy took many secrets to his grave.