05/29/2013 AT 03:30 PM EDT
Superman, buying his outfit in an XL?
"I was fat," says Henry Cavill
, the star of the new Man of Steel
, the latest big-screen take on Superman (out June 14), says about his childhood. "I was Fat Cavill."
A native of Jersey in the English Channel – and a veteran of Showtime's The Tudors
, on which he played Charles Brandon, Duke of Suffolk – Cavill now stands on the brink of stardom in his tight Superman outfit.
But it wasn't always so, the 30-year-old tells Details
for its June/July issue.
In boarding school, Cavill "became an easy target," he says. "I bawled on the phone to my mom four times a day."
His transformation started at age 17, with the decision to become an actor. When casting people descended upon his school looking to assemble a movie version of The Count of Monte Cristo
, Cavill landed a role whose physical rigors helped him to lose 21 lbs.
"I wasn't Fat Cavill anymore," he says. Not surprisingly, the rigors of shooting Man of Steel
truly whipped him into shape.
"We did some of the first shots in Plano, Ill.," he tells the magazine, "and they'd get me up at the crack of dawn, and we'd go to work in this tiny gym at the bottom of the very cheap motel we were staying in. We'd go for an hour, an hour and a half, and then we'd film all day in 100-degree temperatures."
Internally, his childhood experience helped him to understand Superman's psyche. "My version of Superman," he says, "is essentially of a guy who has spent his whole life alone."
For his part, Cavill – which rhymes with "travel" – is not alone. It is reported that he and Gina Carano, the Mixed Martial Arts fighter turned actress, are a couple.