"I think that the queen knew from the start that Diana was going to be difficult, timid, shy," said Nicholl, saying that Kate -- even through two breakups with William -- has never embarrassed the royal family.
"She was the absolute essence of decorum and royalty and dignity and discretion," Nicholl continued, "and I think she earned her brownie points, and what the family have done is they've brought her in."
Jules Knight, a member of the British boy band Blake, said he was friends with William and Kate at the University of St. Andrews in Scotland, and that he is not surprised she has stood up to the pressure so far.
"I think she's conducted herself in the most graceful way," he said. "She hasn't put a foot wrong. She's not the kind of girl you'd see falling out of a nightclub. ... She's a reasonably sensible girl. She's very engaging to talk to, very down to earth and normal."
And, he said, she's the perfect woman for Will.
"She was never seen as the girl who sort of nabbed a prince or anything like that. I think if anything he was sort of seen as the lucky one. ... She's one of those girls who's practically perfect in every way, and I think that we all thought that from the moment we met her, right until now."
William and Kate have been together on and off for eight years, and they have tried to live as normal a life as possible. It's a lesson he learned from his mother.
"Diana famously said to [her sons], 'Boys, there is life outside these palace walls,'" said Nicholl.
She added that Diana took them shopping, gave them money and said they could go buy their own comic books and schoolbooks. She even took them to the movies and McDonald's.
"I think all of these things, even if they weren't aware that they were being taught -- this was Diana transferring her legacy and her humanness," said Nicholl.
This legacy shines on, in William, in his relationship with Kate and in the ring she will wear walking down the aisle and into their future.
ABCNews' Edward Lovett contributed to this report.