Meghan Markle may be a Hollywood actress who captivated the younger Prince of Wales, but palace insiders are warning the American newcomer to not let her star power outshine the royal family.

The “Suits” screen siren, 36, and Britain’s Prince Harry, 33, are set to marry on May 19 in an hour-long ceremony at St. George's Chapel at Windsor Palace.

Leading up to the big day, Fox is airing a two-hour documentary on Friday from British production company Spun Gold, which specializes in royal documentaries.

The film, titled “Meghan Markle: An American Princess,” features interviews with journalist Piers Morgan, Princess Diana’s former butler Paul Burrell, Harry’s biographer Katie Nicholl, and Queen Elizabeth II’s former press spokesman Dickie Arbiter, among others.

Spun Gold founder, Nick Bullen, told Fox News those who came forward stressed that only blood royals are the true stars of the household.

“Kate Middleton has worked this act,” said Bullen. “[Her husband] Prince William is the star and she is the supporting star. Prince Philip has brilliantly walked two paces behind the queen. The queen mother knew you supported the king.

"Meghan, even though she is a famous actress, [her fiance] is Prince Henry of Wales. She will be Princess Henry of Wales. She’s the support. That’s one of the things people in this documentary have warned. Be wary of becoming the star.”

Bullen pointed out the doomed marriage of Harry’s own parents, Diana and Prince Charles. As the Princess of Wales' popularity grew over the years, it reportedly caused a strain between the couple.

Diana and Charles divorced in 1996 after four years of separation. Diana died a year later in 1997 in a tragic car crash in Paris at age 36.

Princess Diana and Prince Charles before their divorce. (Reuters)

“One of the problems from the whole Charles and Diana years is that when Diana became the star, it became a problem for the Prince of Wales,” said Bullen. “… Crowds would shout, ‘We want Diana!’ That didn’t play well. That was one of the warnings that came out of the film to Meghan.

"And what I’ve noticed is that [Markle and Harry] are very much a double act. They’re always together. And I think it will be interesting to see how that plays out over the years.”

Bullen revealed that while Markle, a biracial American divorcee, is changing the look of a seemingly dated monarchy, she will need to play by their rules while settling into her new life as a royal.

“When the royal family was writing these blueprints for royal brides, Meghan couldn’t have been further from their original idea,” he said. “It was blonde, English roses from aristocratic families. [Harry’s older brother] William bent that rule by marrying a girl from a much more middle-class family.

Britain's Prince Harry poses with Meghan Markle in the Sunken Garden of Kensington Palace, London, on Nov. 27, 2017. (Reuters)

"And Harry is taking it a stage further… History proves with the most recent royal wives of Windsor that if you don’t stay in your lane, you can fall down. Diana became the star and that marriage didn’t work. Fergie was fun, free loving and a breath of fresh air. But that didn’t work.”

But unlike Diana or Fergie, Markle is getting hands-on help from Harry, who has always been by her side for every royal outing since their November 2017 engagement.

Bullen said Burrell, who started off his career as the queen’s footman before working for Diana and her sons as the family butler, isn’t surprised by Harry's determination to protect his fiancee from media scrutiny.

“Harry was born into a marriage that was falling apart,” said Bullen. “And it’s a marriage that played out on the world stage… The dirty laundry is right out there… Then at age 12, he has to walk behind his mother’s coffin in front of 2 billion people watching from around the world. He has to shake the hands of people who are weeping, who haven’t even met his mother. And yet he has to hold it together.

Prince Harry (center) with his brother Prince William and father Prince Charles at Princess Diana's funeral. (Reuters)

"That plays forward to today in terms of why Harry wants to be so protective of Meghan. When the British press were frankly being racist about her arrival to the U.K. and into the royal family, Harry issues a statement essentially saying 'back off.' And they did back off. He couldn’t protect his mother as a 12-year-old boy, but he is determined to protect Meghan. And it comes as no surprise that he and his brother have a big distrust of the media.”

But if Markle really wants to fit in with the royal family, the California girl will need to do more than giving up her thriving acting career and popular lifestyle blog, which she has already done.

Markle must also learn how to saddle up like a pro.

Arbiter, who closely worked with the queen in Buckingham Palace for 12 years, stressed Markle must love horses to fit in.

A young Prince Harry herding bulls. (Reuters)

“This is a very horse-driven family,” said Bullen. “Diana didn’t go well with horses. That was the beginning of that fairy tale not ending terribly well. Sophie, Countess of Wessex who is married to the queen’s youngest son, had to learn how to ride.

"You have to like horses to ride with the queen. And so much of the royal diary is built around the equestrian world… One of the most important events from the queen’s diary for the whole year is the Royal Windsor Horse Show. And that will be happening on the weekend before the royal wedding… The queen has never, ever missed it. It’s really important to her.”

Despite the many lessons Markle must learn to become a successful American royal in the House of Windsor, Bullen said insiders are hopeful the marriage will last.

“I think everyone wants it to work because Prince Harry is adored around the world,” said Bullen. “He’s the little boy who lost his mother at 12. He was a slightly off the rails teenager who became a war hero fighting on the frontlines… We want the fairy tale, which has had some tragic lows across the last 30-40 years. The overriding message of the documentary is 'fingers crossed.'

Meghan Markle with Prince Harry after their engagement announcement. (Reuters)

"But those in the documentary warned that this world is not without its pitfalls," Bullen said. "You marry into a world that’s diamonds and furs and castles and servants and literally knights in shining armor. But it all comes with a price. Are you ready to pay that price?”

"Meghan Markle: An American Princess" airs Friday, May 11 on Fox.