The Princess Royal praises her sister-in-law, Queen Camilla, as she appears in a new film documenting the King's coronation year.
Princess Anne, 73, is among the commentators in the BBC film, which will air on Boxing Day.
"I've known her a long time, off and on," The Princess Royal says of the Queen: "Her understanding of her role and how much difference it makes to the King has been absolutely outstanding.
"This role is not something that she would have been a natural for, but she does it really well. And she provides that change of speed and tone, she's equally modern."
In the 90-minute documentary, Anne is seen among the royals as they prepare for the historic coronation ceremony during rehearsals at Buckingham Palace and Westminster Abbey.
The Princess Royal was given a starring role as the King's Gold Stick-in-Waiting on the day, riding on horseback alongside the Gold State Coach as the newly-crowned Charles and Camilla made their way back to Buckingham Palace after the service.
In another scene, the King is seen kissing his sister Anne's hand as the royals prepare to greet the crowds from the Buckingham Palace balcony.
Charles was just four when he attended Queen Elizabeth II's 1953 coronation. Anne was too young to attend the ceremony at the age of two, but she later appeared on the balcony with her parents and brother.
Just a few months shy of his 74th birthday, Charles became King following the death of his mother at Balmoral on 8 September 2022.
He became the oldest person to accede to the British throne, after having been the longest-serving heir apparent and Prince of Wales in British history.
Talking about the succession, Anne goes on to say: "To be honest I’m not sure that anybody can really prepare themselves for that kind of change… not easily. And then the change happens and you go 'OK, I now have to get on with it'.
"Monarchy is a 365 days a year occupation, it doesn’t stop because you change monarchs, for whatever reason."
Other highlights from the documentary show the moment the King jokes about having "sausage fingers" during a rehearsal with his son, the Prince of Wales.
Prince William was filmed closing a small clasp on the King's robe, saying: "On the day, that’s not going to go in."
Charles replied with a smile: "No, you haven’t got sausage fingers like mine."
Charles III: The Coronation Year airs on Tuesday 26 December at 6.50pm BBC One & iPlayer
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