Harriet Walker: Kate would do well to harness some of Diana's irreverence

The public take Prince William at face value, as a bit of a gent with a good heart who likes messing around in planes

The spectacle of the newly married Duke and Duchess of Cambridge driving down The Mall on Friday in an Aston Martin was a handy metaphor for what Wills and Kate will now do to the Royal Family: drag them into the 21st century like the balloons and cans attached to the car's rear bumper.

There are a couple of ways they can do it. First, by tempering some of the stuffiness with the current mindset for which they have become known, and to which many components of their nuptials pointed. Second, by deploying Princess Catherine, in all her down-to-earth but serene, commoner charm.

It's a fine line to tread. We want to see her dancing at Mahiki, but not to watch her stumbling into a taxi, à la Jordan. Princess Kate needs to keep her feet of clay tucked well under her skirts. She's not Diana, we are told , but she would do well to harness some of her irreverence; she has dotted the i's and crossed the t's throughout her relationship with William.

Kate must open herself up a bit more. That's not to say admitting her darkest secrets, but tabloids and hoi-polloi alike enjoy feeling linked to their icons on a human level. In choosing a bullying charity to benefit from donations at her wedding, Kate has already bared a little of her soul. Now it's time for her to move on to trickier issues and shine that smile into some of the darker sectors of society.

Perhaps Kate could pick up Diana's work with Aids; HIV in the Third World would be a modern and meaningful choice. Domestic abuse charities perhaps, or getting involved with feminist groups and "reclaim the night" marches – these are the issues most women of the Duchess's age are concerned with, and the key to her success and popular appeal is appearing realistic. That is why William is so loved – because the public take him at face value, as a bit of a gent with a good heart who likes messing around in planes.

Kate also needs to maintain that reputation for looking sleek and groomed, and to cultivate her penchant for the high street. Look how much more popular Sam Cam is than Sarah Brown or Cherie Blair; the public love the opportunity to spot someone important wearing Zara.

So it's not hard really – Kate needs to be a style icon and subversive saint. Friday's wedding may have had all the hallmarks of a fairy tale, but pigs haven't started flying yet.