The King jokes about his "sausage fingers" and is seen telling the Archbishop of Canterbury Justin Welby to "jam" the crown on his head in a new documentary about his coronation.
His Majesty reveals a self-deprecating sense of humour as he chats to his son Prince William during a rehearsal at Westminster Abbey.
As the monarch and his heir practise putting on the Imperial Mantle, the Prince tries to fix the clasp, telling his father: "On the day it’s not going to go in, is it?"
Laughing, Charles replies: "No. You haven’t got sausage fingers like mine."
In another touching father-son moment, William tickles his father’s cheek after first kissing his right cheek then deciding: "Your left cheek is better."
And the Archbishop jokes to the Prince: "If you ever get lost Sir in this, you just look confident and bow."
In another clip, filmed during a rehearsal in the ballroom at Buckingham Palace, the King discusses how the Archbishop should place the priceless St Edward Crown on his head.
"You have to jam it on," he says. Pointing to his eyebrows, he adds: "It has to come down to here first."
© BBC/Oxford Film and Television"I don’t want to break your neck Sir!" replies the Archbishop. "It might ruin the service."
"I promise you, it’s so huge, it’s got to be on right and I can’t do anything about it," the King says.
Elsewhere the Queen jokes, "I slept so well" after military bands spent the night rehearsing on The Mall outside Clarence House.
And when told the King has to give permission, by nodding, for the Archbishop to crown her, she jokes: "Don’t bother to look, I’m very happy."
© GettyHer close friend and Companion of Honour, Lady Lansdowne says: "She has got a great twinkle and it comes out very readily and it puts people at their ease very quickly. She knows when to be serious. And she knows when to wink at a bishop, but when not to which I think is a rather endearing quality."
The extraordinary behind-the-scenes footage appears in Charles III: The Coronation Year, which airs on Boxing Day.
© BBC/Oxford Film and TelevisionWith commentary by the Princess Royal, the Archbishop of Canterbury, the Queen’s sister Annabel Elliot and her close friend and Companion of Honour Lady Lansdowne, the programme follows the King and Queen in the run up to and in the months after their historic coronation.
The feature-length film by Oxford Films is narrated by Helena Bonham Carter – who plays Princess Margaret in The Crown – and directed by royal author and journalist Robert Hardman.
Charles III: The Coronation Year airs on Tuesday 26 December at 6.50pm BBC One & iPlayer
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