How Kate Middleton's 'Immaculate' Appearance at Trooping the Colour Took a Toll on Her: Royal Expert

Royal expert Katie Nicholl tells ET how Kate Middleton's 'immaculate' appearance impacted her.

Kate Middleton's Trooping the Colour appearance came amid a challenging time for the Princess of Wales. Following Kate's stunning showing at King Charles III's birthday parade, ET spoke with royal expert Katie Nicholl about how the public display affected Prince William's wife amid her cancer treatment.

"The princess has this sort of swan-like quality about her -- she's sort of gliding on the surface -- but beneath the water, I think often she's paddling quite frantically," Nicholl told ET. "I can only imagine that a huge amount of preparation and time and effort and energy will have gone into Saturday's appearance."

"She was on her feet for a long period of time," she continued. "There was a moment where she was watching the parade... and was offered a seat, which she gratefully accepted. That's just a bit of an acknowledgement that she is not as strong, not as fit, as she was before she went into this treatment."

Even amid her health challenges, Nicholl noted that Kate "looked impeccable" at the royal event, adding, "If you didn't know that she was going through cancer treatment, you would never have guessed."

"I think a day like Saturday, where she knows the eyes of the world are on her, there is a huge amount of scrutiny that will take its toll," she said. "I think we can probably imagine that she probably had a long and well deserved rest afterwards. I don't think we can expect to see her in public for a little while. That's going to take some recovery from."

Kate Middleton at Trooping the Colour - Neil Mockford/GC Images

As for how her appearance at Saturday's event came to be, Nicholl said that attending was Kate's "decision alone."

"There was no pressure on her from anyone. She really wanted to do this," Nicholl said. "She turned a corner in her treatment... When she turned that corner, I was told that Trooping was a goal for her. She was given the sign-off by her medical team. She spoke to her father-in-law about attending and to Prince William. They were fully behind her."

Nicholl added, "Charles has been a great pillar of support to Catherine when she said that she'd really like to be there at Trooping. He was 100 percent behind it. The idea that she was going to upstage him didn't even cross his mind. He just wanted his daughter-in-law by his side."

What followed was noting "short of immaculate," Nicholl said of Prince George, 10, Princess Charlotte, 9, and Prince Louis', 6, mom.

"I think clearly a huge amount of effort went into her appearance behind the scenes, and it wasn't just about how she looked. It was how she held herself. It was the pride that she took in being there, the way she shepherded her children into the carriage and up onto that balcony appearance," Nicholl said. "It was obviously very, very important for her to be there on Saturday, and I think she went to great lengths to make sure that she could be, and to do it all with that wonderful smile on her face."

Prince William, Kate Middleton, Prince George, Princess Charlotte and Prince Louis attend Trooping the Colour. - Mark Cuthbert/UK Press via Getty Images

Throughout the day, Kate was seen alongside her father-in-law, who's also battling cancer.

"At last year's Trooping, you had William and his father shoulder by shoulder. This year, it was Kate who stood next to the king. I think that was very deliberate," Nicholl said. "The cameras caught them in a bit of an exchange, where they seem to be talking very warmly, very fondly. I'm hearing that always close relationship between Kate and her father-in-law is now even closer... They are both going through cancer treatment. They're both going through good and bad days, and therefore they're able to relate to one another and have some sort of understanding that probably they can't have with any other members of the family." 

Prince William, Kate Middleton, Prince George, Prince Louis, Princess Charlotte, King Charles III and Queen Camilla at Trooping the Colour. - Gareth Fuller/PA Images via Getty Images

Following the successful appearance, Nicholl said "there's a great sense of pride" in Kate.

"I think she felt very much that she needed to be there," she said. "The children looked so happy and relaxed having their mother there... It was important, I think, for the Princess of Wales to be there to support her father-in-law, to be there for her family, and to be there for the nation to send out a really important message that she's doing OK."

While the showing should "give us all a great sense of optimism" about Kate's health, it doesn't mean she'll be back to full-time work anytime soon.

"She was very honest in that message. She said there were good days, and there were bad days when she was tired... I think we have to assume that Trooping was a good day, but she is still going through cancer treatment," Nicholl said. "She's spoken about this need to be able to go through this period privately for her own recovery, but also for her and her family to have that much-needed privacy."

In the coming months, Kate is expected to "attend a handful of engagements" as long as she gets a sign-off from her medical team and feels well enough. Wimbledon, Nicholl mused, might be a good event for the Princess of Wales to attend.

"I wonder if we might see her at Wimbledon," she said of the July tennis tournament. "She spoke in that very personal message about wanting to be able to do the things that bring her joy. I think that's being in nature, that's being with her children, that's taking part in everyday school life, but tennis, don't forget, is something that the princess absolutely loves."

"She takes great pride in her position at the Lawn Tennis Association. Wimbledon is a firm fixture in her diary. Every year we've seen her attend with her family in the past," she continued. "It wouldn't surprise me at all if we see her pop up at Wimbledon, because I think that is something that will bring her joy, and a huge amount of her recovery depends on her being happy."