In the latest issue of Radio Times, they interview a bunch of Potter stars including Matthew Lewis. See a scan from the issue in the gallery! In the article, he says:
I was a huge fan of the books. My mum always reminds me of the time I begged: “If they make a film, will you take me to see it?” Little did I dream I would be in it!
“In the first couple of films, I looked like Neville was supposed to. But when it came to the third film, I suddenly got taller and slimmer, so they had to make me geekier. I had a fat suit and false teeth. I tried to be professional about it but I was 15 and on set with girls – there were definitely times I wished I were a more macho character! In this last film, we’ve got rid of the fat suit and Neville has some really cool action scenes. Playing the hero is definitely something I could get used to.”
Thanks to SnitchSeeker!