WizardUniverse has added their interview with Rupert to their website.  If you recall they also interviewed Dan and Emma.

In the future when you look back on the making of this film, what are some funny or strange moments that you’ll remember about the making of it?
GRINT: Getting my bum cast. They were making a chair for the broomstick to make it more comfortable and have my shape. I was wearing these skinny trousers and I had to sit on this chair and they managed the exact shape of my bum. They turned it into a really comfortable chair.