In an interview with BBC, Nixon —who was the first person to interrogate Saddam at length —said that former Iraqi President was the most "secretive and suspicious" man he has ever met.
“When Saddam Hussein was captured in December 2003, the CIA required a specialist who could identify and interrogate him for information. That person was John Nixon,” John added.
There had been rumours at the time that Saddam Hussein had numerous body doubles, but Mr Nixon - who left the CIA in 2011 - says "there was no doubt in my mind as soon as I saw him, that it was him".
"When I started talking to him, he gave me the same look he had on a book that had sat on my desk for years. Surreal doesn't come close."
Mr Nixon took on the role of interrogator and was the first person to question Saddam Hussein at length, doing so across a number of days.
"I had to keep pinching myself that I was questioning the most wanted man in the world. It seemed ludicrous," he says.
Mr Nixon, author of Debriefing the President: The Interrogation of Saddam Hussein , describes the former leader as a "mass of contradictions".
He saw "the human side" of Saddam Hussein , he says, in great contrast to the depiction presented by US media.
"He was one of the most charismatic individuals I've ever encountered. When he wanted to be he could be charming, nice, funny and polite."