Diana’s private voice and presence coach Stewart Pearce said the portrayal of the princess in the film was “boring” and failed to capture the true spirit of her royal friend.
Pearce admitted that the breathy and stressed way of the dialogue often meant he couldn’t understand parts of the film.
Although a fan of Stewart’s talent, he was not impressed with Spencer.
Its main criticism was that the film did not portray the mother of princes Harry and William as having “flow and grace”.
The respected author adds that the film, directed by Pablo Larrain, fails to “communicate his inner strength”.
While Hollywood insiders see Stewart as a candidate for an Oscar nomination, Pearce feels very differently. The 68-year-old said he was not shocked the two-hour feature cost just over £ 1.5million ($ 2.1million) in the past few days, despite opening on 996 screens across the country.
He had expected some viewers to come out of cinemas before the end of the “immensely gray” drama.
Speaking exclusively on camera, Pearce admitted, “I was challenged in a really interesting way. It was not comfortable to watch.
“Kristen’s rhythms were very jerky and very breathy. It seemed that many actors had been encouraged to speak in a conspiratorial manner.
“They were all using these very, very low voices and moving so fast that it was often very hard to hear what was going on.
“My work is about how we don’t judge the characters we play, but we find points of empathy so that we really care about what is happening on screen. If the actors don’t care who they’re playing, it’s automatically communicated to the audience.
“Now the audience won’t necessarily get the subtlety of that, but what they’ll do is turn off their screens or leave the movie theater because the vibration won’t delight them.”
Pearce observed that Diana was “all about flow and grace”, but claimed that Stewart gave a “very choppy and choppy performance.”
The independent film is said to have cost more than £ 13.3million ($ 18million) to make, including marketing costs.
Pearce spoke exclusively as he prepares to tour the United States to promote his bestselling book Diana: The Voice Of Change.
Diana met Pearce in 1995, in the strictest confidence, to perfect her voice, balance and expression.
The veteran vocal coach, who also worked with former Prime Minister Margaret Thatcher, helped the princess technically refine her voice, using mental exercises to improve her confidence, independence and resolve.