From curtsying to Queen Elizabeth to calling her "Your Majesty", U.S. actress Meghan Markle will have to learn her royal lines when she marries Prince Harry and joins the ranks of the British monarchy.

Unwritten rules govern how the royals should behave and the public act around them. While many antiquated protocols have fallen by the wayside, there is still some etiquette Markle will be expected to follow after her wedding to the queen's grandson on May 19.

"The problem is that she's got to remember that, as a member of the royal family, she represents the family or, as it's been called, 'the brand'," said Grant Harrold, who served as a butler to Harry himself while working for his father Prince Charles, and now provides expert guidance on the subject.

"So, I think there is quite a lot pressure to make sure she gets it right because the last thing she wants to do is do something wrong or make a mistake and it ends up becoming front page news - and then it's embarrassing for her and for the royal family," Harrold told Reuters.

For someone who grew up in Los Angeles, life behind palace walls - where butlers, footmen and members of the royal household, often dressed in smart traditional uniforms with scarlet waistcoats, discreetly go about their jobs - could scarcely be more different.

FLUMMERY

"I think Meghan will cope - but she will find some of the flummery difficult to bear," said Andrew Morton, who has penned a biography of the bride-to-be.

"This was a girl who was a gender equality advocate for the United Nations - having to bow and curtsy to the queen and even (her future sister-in-law) Kate Middleton in private occasions."

Harrold says the formal protocol surrounding the royals, such as when to bow or curtsy, and to whom, and how to eat at royal banquets, was mostly set by the 17th century French king Louis XIV before being embraced by other sovereigns.

"Etiquette and protocol is really important to the royal family. It's been important to royals for centuries," Harrold said. "Those rules are there ... one - to make them understand what's expected of them, but also - so they understand what to do and what not to do."

As such, until Markle marries Harry and assumes the style "Her Royal Highness", she should, strictly speaking, curtsy to all the other royals with such a title, such as Kate, the wife of Harry's elder brother Prince William.

Strict protocol also dictates that she should walk backwards from the queen in her presence, although Harrold says you would not see this in public. But as the monarchy evolves and modernises, these conventions are not as important as they once were, something the royals themselves acknowledge.

"There are no obligatory codes of behaviour when meeting The Queen or a member of the Royal Family, but many people wish to observe the traditional forms," the royal family's website says - before explaining how to bow and curtsy and address the Windsors.

Those who breach the unspoken rules can sometimes expect a frosty reception.

Last year, the governor general of Canada admitted breaking protocol by touching the queen's arm during an engagement in London and Australian prime minister Paul Keating was dubbed the "Lizard of Oz" by the British press after he appeared to put his arm around the monarch's shoulders in 1992.

"QUEEN DOESN'T CARE"

"Members of the royal family do curtsy to the queen," said royal historian Hugo Vickers, an adviser for the 2011 Oscar-winning film "The King's Speech".

"The queen doesn't care actually if people bow or curtsy or not, but I guess if it's somebody who ought to know, she probably would. But she's not going to tick people off for not doing it."

He said the protocol was very well defined and would not be difficult for Markle to learn.

One convention that Markle will particularly have to be aware of that the British royals are expected to steer clear of making any overtly political statements in public.

"She's got to be more careful about how she's photographed and what she's doing," Claudia Joseph, author of "How to dress like a Princess", told Reuters.

"She will have to curb what she says, she's not going to be able to be as political as she was beforehand - and that might be trouble in the future."

Harrold said her future husband Harry and the other royals would help to steer Markle through any minefields.

But another etiquette expert, Liz Brewer, had a warning:

"She needs to remember now that she is no longer an actress acting a part - although she is a very good actress. She is now part of 'Brand Britain', and as such, everything she is doing will be geared towards that."