Helena Bonham Carter hired a psychic to contact the late Princess Margaret before playing her in The Crown.
The 52-year-old actress enlisted the help of a clairvoyant to speak to Queen Elizabeth's sister - who died in 2002 aged 71 - from beyond the grave to ensure she researched the role with "as much depth as possible".
A TV insider said: "She always likes to research her roles with as much depth as possible - and is always prepared to go that extra mile.
"So if that means using someone with supernatural powers to speak to a character who is now on the other side then Helena has no problems with doing that."
And the Harry Potter star - who will appear alongside Olivia Colman, who will replace Claire Foy as the queen, in the third series of the Netflix drama - is said to have received "positive feedback" from the princess.
The insider added to The Sun newspaper: "She is naturally spiritual and open-minded and is delighted she's had such positive feedback from the late royal."
Helena is set to take over the role from Vanessa Kirby, who played Elizabeth's sister in the first two seasons of the show, and admitted she and her co-stars Olivia and Tobias Menzies - who will replace Matt Smith as Prince Philip - are feeling the pressure to follow on from the "successful" first two series.
She recently said: "I think we're all ... we're completely terrified.
"I think also because the first two seasons were such a success, we have the onus of inheriting the responsibility of doing justice to all these genuinely famous people, and then on top of it, inheriting them from this previous generation of actors who've done such good jobs.
"None of us look at all like our previous generation.
"We don't actually look like our real people either. I don't look like Margaret. I don't think Olivia looks particularly like the queen, but it's interesting. We just have to try and create some sort of essence.
"The good thing is that all the characters are so multifaceted, so we will probably capture different bits."