Dudley doesn't feature in the sixth Potter film, so there was a long gap between playing the butterball bully in the fifth, aged 16, and shooting the seventh film, this year. In the meantime, he had changed almost beyond recognition, to the producers' alarm. "They did this double-take, 'Oh my God, we are going to have to do something,' and I felt very guilty. They could have recast, but instead they padded me out."

This focus on his career is, no doubt, the result of coming from a family riddled with actors. His grandfather, Patrick Troughton, was, for many, the greatest Dr Who. His uncles, Michael and David Troughton, and his cousin Sam, are all actors. His illustrator mother, Joanna Troughton, was keen to give Harry a chance, too. "She sent off my photos to the producers of the Potter films when I was 10, thinking I might be an extra. A year later I was asked to read for Dudley. When I got the part, I thought: 'Ooh, I'm in a film', not that I was cast as fat and ugly.'"

Wasn't being associated with the hateful Dudley a curse? "I kept quiet about it. Nobody ever recognised me."

He minded more about not being one of the Hogwarts' children. When people asked him about the films, and what Michael Gambon or Julie Walters were like, he would have to say, "Dunno, I've never met them." But, stuck in Privet Drive, he did get to work with Richard Griffiths and Fiona Shaw, who play Uncle Vernon and Aunt Petunia, Dudley's parents. "Just watching them was an education. Fiona taught me a Caliban speech, which I later used to get into drama school."

Compared to Daniel Radcliffe, whom he once envied, Melling now realises that he was lucky not to be one of the Hogwarts' leads. His career got off to an early start, but without the irksome trappings of fame, or the danger of being typecast. "I can now shed the child-actor thing, like the fat, and start a new career, because no one sees me as Dudley."