The Princess of Wales admitted to feeling nervous ahead of tomorrow's historic Coronation but would try to remain calm on the outside.
The surprise appearance shocked royal fans who had been lining The Mall as they surged to snap selfies with the royals and shake hands with them ahead of tomorrow's historic crowning of Charles and Queen Camilla.
Speaking to one wellwisher, While Kate revealed how Prince George, Princess Charlotte and Prince Louis are feeling about the Coronation.
"They're really well, thank you," she said. "Excited, a bit nervous with a big day ahead."
Kate then explained she want to appear "relatively calm on the outside" even if there were nerves on the inside - and likened herself to a swan.
When asked how she and William were feeling, she explained: "All good. Hopefully a bit like a swan, relatively calm on the outside and paddling on the inside.
On Charles, she said: "I'm sure looking forward to it as well. It's nice to be out here saying 'hello' to everyone who has stayed out for so long."
The cute exchange happened during a spontaneous walkabout by King Charles, Prince William and Princess Kate. Following their morning engagement, the royal trio stopped to chatted to royal fans who are already camped out on The Mall.
The road leading to Buckingham Palace was cleared and royal fans eagerly lined the route, having likely figured out that they may well catch a glimpse of the King due to the security and media presence.
The trio took time to shake hands with members of the public as people cheered, took selfies and sang God Save the King.
The King went to one side of The Mall while William and Kate went to another to greet well-wishers.
One woman told Charles "Love you Charlie" while others passed on their congratulations to him. Kate and William posed for selfies with fans, while Kate even appeared to join a fan in a video call before also speaking on another fan’s phone before handing it back.
William spoke to an American woman from Phoenix, Arizona, who was planning to camp out overnight.
Applauding the royal fan, the Prince of Wales joked "there's not much sleeping going on. All sat in your chairs." He then added: "There will be a good party atmosphere."
When one well-wisher asked the King if he was nervous about the Coronation tomorrow, he aughed good-naturedly and held his hands in the air.
Another fan Theresa Iredale from Banbury, Oxfordshire, whose 66th birthday is tomorrow, was among those who congratulated Charles.
She said: "I said congratulations. He asked if I had been here all night. I just told him I was so happy for him."
Margaret Tinsley, 81, from Gillingham, Dorset, was reduced to tears after shaking hands with the King and telling him: "We are so proud of you, sir."
She said: "I also said God bless you, sir and then someone said I’d been camping out since Tuesday. The King took both my hands and said, Have you really? He is so kind."
Maureen Bennett, a retired financial adviser from Brighton who was wearing a Union Jack bowler hat and a Coronation t-shirt, shook hands with the King, William and Kate.
She said: "The King said you are being a bit discrete, referring to my outfit. I said good luck for tomorrow and he said that’s very kind of you."
Her friend Lucy Clark, 47, from Whitstable, Kent, said: "William said he wanted to hear the party vibe like there was for his wedding to Kate. I told him there will be a party vibe going on and he said, Good."
Another fan who congratulated Charles also told him: "You look younger in the flesh."
As the King walked along one side of The Mall and then the other, the crowds shouted "Three cheers for the King." One well-wisher held a banner featuring picture of the late Queen and the words "Mummy would be proud".
As Charles passed along and chatted with members of the crowd about what it was like camping put, he told one group: "I hope you all don’t get too wet."
After spending time meeting wellwishers, they returned to their cars to head back into Buckingham Palace.
Earlier, the King took part a final coronation rehearsal at Westminster Abbey, where a 2,300-strong coronation congregation, and a television audience likely to be in the tens of millions, will watch him being crowned on Saturday.
The King has a busy day with a series of diplomatic duties and has been joined by working royals at a special lunch for realm prime ministers and governors general at Buckingham Palace.
Charles, who is head of the Commonwealth, will also attend a Commonwealth Heads of Government leaders’ meeting and garden reception at London’s Marlborough House the same day.
Also on the eve of the coronation, the King and the royal family will host a glittering reception for foreign royalty and other overseas dignitaries at the Palace.