Andrew Morton, who appears in the fifth season, secretly collaborated with the late princess at a time when her marriage to the Prince of Wales was at breaking point.
Diana reportedly answered his questions on cassette tapes passed back to him through the intermediary Dr James Colthurst, her good friend.
Morton, who wrote Diana: Her True Story, first published in 1992, based on the tapes (and was a consultant on the new show), told Vanity Fair in an interview that elements of the programme are accurate to his experience.
But he was surprised that one scene was omitted.
"One scene I’m surprised they didn’t do was James letting me listen to the first tape in a working-man’s café in Ruislip in north London," he said, telling the magazine that Colthurst wouldn't let the tapes out of his sight to safeguard Diana.
"All these working men were sitting around eating their eggs and bacon. I put the headphones on and I was transported to another world, a world of bulimia, Camilla, and things which I’d never heard of.
"It was literally like entering a parallel universe where you had a secret and that secret was quite dangerous.”
Morton also opened up about other related events that unfolded, including Colthurst once being knocked off his bike by a vehicle, causing one of the tapes flying away, and returning to his own office one to day to discover it had been broken into and a camera had been taken. This made him paranoid enough to only communicate with his secret partner via payphone.
"Looking back, I think we had every right to be paranoid given what happened with [the leaked royal phone calls] Squidgygate and Camillagate," said Morton, telling Vanity Fair that the real-life experiences were sometimes "even spookier" than in the episodes.
'Squidgygate' refers to a leaked transcript of a phone call between Diana and James Gilbey in which he expressed his affection for her, calling her 'Squidgy'. 'Camillagate', or 'tampongate', meanwhile, refers to a leaked transcript of a phone call between Charles and Camilla, with them expressing their desire for one another and him joking about being a Tampax to be closer to her.
“It was a time where you didn’t know if you were being followed, where you didn’t know if you were being watched. Diana was worried about her phones being tapped," he added.
Morton never actually met the princess, partly to avoid being photographed together and raising suspicion. He only confirmed her participation in his book after her death in 1997.
And on Elizabeth Debicki's portrayal of Diana, he said, "It was like going back into the room 30 years ago. I was blown away by how she got every nuance of her character.”
Watch: Charles and Diana at war: What’s real and what isn’t in S5 of ‘The Crown’