"I think that really is the long-lasting legacy that Diana has left William and Harry," Arbiter said. "She took them outside the palace walls."
Patrick Jephson, Princess Diana's chief of staff for six years, met William when the prince was six years old.
"She made sure that they experienced things like going to the cinema, queuing up to buy a McDonalds, going to amusement parks, those sorts of things that were experiences that they could share with their friends," Jephson said.
Diana had to balance the clash between these normal experiences and the fact that her sons' lives would in many ways be anything but normal.
"It was a very difficult dilemma for Diana to prepare them for the very distinctive, unique life that they have had to lead," Jephson said. "And she did it very cleverly, I think."
For example, he said, Diana took them to hospitals and homeless shelters to introduce them to the work she had found so fulfilling in her own public life -- and that would be their destiny.
Wharfe recalled her taking seven-year-old William to a homeless center.
"This was done completely out of sight of any camera or media. This was Diana's way of actually saying to William, 'Listen, it isn't all what you think it is living at Kensington Palace.' That was a quite a brave thing on Diana's part."
As adults, both William and Kate have followed in Diana's footsteps. Last year alone, Kate made 111 appearances, many in support of charitable causes. William still actively supports Centre Point, the homeless shelter he first visited with his mother. Last year, William spent the night on the streets of London to highlight the plight of the homeless.
"She played a huge part in my life and Harry's growing up, in how we saw things and how we experienced things," William said in a 2012 interview with ABC News' Katie Couric.
"She very much wanted to get us to see the rawness of real life. And I can't thank her enough for that, 'cause reality bites in a big way, and it was one of the biggest lessons I learned is, just how lucky and privileged so many of us are -- particularly myself," William said.
Diana also showed by example how to handle encounters with the public, beginning with William's first official public outing, at 10 years old.
"Even as an adult that might have been quite intimidating," Jephson said. "But William squared his shoulders, he went and did his duty, he did his job, and he made sure that everybody who met him that day had a memory to treasure forever."
"Diana said to William, It only may be 10 seconds out of your life, [but] it could be years of happy memories for the person you meet," Jephson went on. "And that sense of respect for people in the crowd is something that I think we can see Diana has passed on very successfully to her children."
William told Couric, "She really minded about who she met and how she did the engagements. And she wanted to give everyone a certain amount of time so that they felt they were actually engaging with her. And I thought that was a very good learning point for me."
Diana had a common touch that made her loved, particularly by children. She always crouched to talk personally -- eye to eye.
"Diana was the first member of the royal family to do this," said Ingrid Seward, editor of Majesty magazine. "The royal family used to say that everyone had to be deferential to them. But Diana said, 'If someone might be nervous of you or you're speaking to a very young child or a sick person, get yourself on their level."