I think getting Mary to do it was a masterstroke. And also the fact that she was asking quite personal questions in an informal setting and getting informal answers. I am sure those questions were primed to a large extent, but it meant that they allowed a sliver of their personal lives out - cooking with the kids, George's treasure trove in the garden, how William's mother influenced him and this time of year is hard without her etc etc. Those sorts of questions don't work in a formal interview and in formal interviews it's hard to seem spontaneous. It was well done. Even the spinach soup anecdote was probably lined up ahead of time, but nonetheless sounded true and made Kate seem more real and personable.
The bit where she says something to a group of women about babies coming at Christmas was also good. I thought it would have been nice to see more of that - it almost looked like a normal light gossipy group of women.
There's a bit at the end where william puts his hand on her shoulder and she seems to shake him off which did jar for me. But I think it was just edited so that we couldn't see the context.