The actor - who had to strip off for a scene in the bath in the new comedy 'Wild Target' - admitted he wasn't very body confident about wearing just underwear in front of his co-star.
He told BANG Showbiz: "It's quite embarrassing where I had to stand in the bath, I had a jock strap kind of thing, it was weird because it wasn't water, it was milk... to make the water look cloudy so you couldn't see anything - we were in a bath of milk.
"Thankfully there weren't many people on set just me and Bill. He didn't really say very much, but yes it did feel just a bit uncomfortable for the both of us."
While the 21-year-old star - who is best known for portraying wizard Ron Weasley in the 'Harry Potter' films - felt awkward about being near-naked in front of his co-star, he isn't worried about audiences seeing him naked - even though he admits his body isn't in the best shape.
He added: "It's embarrassing, I've never been to a gym or really done any sports... but I'm not lazy as I'm always working or busy doing something."