Helena Bonham Carter says she spoke to the late Princess Margaret to ask if she could play her in The Crown.
The 53-year-old actress says the deceased royal gave her the seal of approval via a psychic, telling her she was “better than the other actress” being considered for the part.
Olivia Colman will play Queen in the third series of Netflix’s lavish period drama.
Speaking at The Cheltenham Literary Festival, Bonham Carter said Princess Margaret also told her to “brush up and be more groomed and neater”.
She also says she warned her to “get the smoking right”.
Princess Margaret, who was a lifelong smoker, used a cigarette-holder which the actress says “was as much a weapon for expression as it was for smoking”.
At just five feet tall, the actress says it was a way to give the impression of being taller.
She added that while the princess didn’t mind being “number two” to the Queen, she did resent being so small.
Bonham Carter, who is taking over The Crown role from actress Vanessa Kirby, says she also consulted an astrologer, former ladies-in-waiting and a graphologist (someone who studies handwriting) before stepping into the role.
During her stage interview with broadcaster Emma Freud, the Harry Potter actress revealed she had a personal link with the princess, as her uncle once dated the royal.
She said former Liberal MP Mark Bonham Carter dated her when he was a Grenadier Guard at Windsor, describing them as a “really dashing couple”.
She also claimed that Margaret was a much misunderstood character, with her vulnerability often mistaken for rudeness.
Bonham Carter says she did meet Princess Margaret several times, with the royal once commenting: “Oh Helena … You are getting better at acting, aren’t you?”.
Season three of The Crown will be released on 17 November on Netflix.