By Nicole Sperling
Updated May 22, 2014 at 05:00 AM EDT
What If
Credit: Caitlin Cronenberg

In the upcoming modern rom-com What If (Aug. 1), Daniel Radcliffe trades his wizard robes for jeans — and a bit of nudity.

But prepping for his first romantic lead required more than just dropping his drawers. In the film, from director Michael Dowse (Goon, Take Me Home Tonight), Radcliffe, 24, had to get comfortable acting in modern times. After Harry Potter and period films such as The Woman in Black, What If — about a med-school dropout (Radcliffe) who falls for an already attached art director (Ruby Sparks’ Zoe Kazan) — is Radcliffe’s “first foray into the 21st century,” he notes. To prep, he boned up on his genre faves: When Harry Met Sally…, It Happened One Night, and Arthur with Dudley Moore.

“It’s really, really funny,” says the actor of Arthur. “In so many romantic comedies the humor is kind of lame.”

Though Radcliffe doesn’t share many characteristics with his less ambitious character, the two do both sport one unfortunate quirk. “On the second page of the script, he corrects somebody’s pronunciation of a word, and I thought, ‘Oh God, I’m this a–hole,'” says Radcliffe, recalling the first time he read Elan Mastai’s script. “I’ve gotten better, but it’s like I’ve got some version of grammatical Tourettes. I can’t hear someone make a mistake without wanting to correct them.”

The actor has dropped trou for stage productions of Equus, as well as last year’s movie Kill Your Darlings — including a dream-sequence skinny-dip in NYC’s Hudson River later cut from the film — so he wasn’t put off by What If‘s naked swim. The bigger challenge: braving Toronto’s chilly beaches. “We were really dreading it,” he says of the fall 2012 shoot. “It had been cold a few nights before, but it got beautifully warm. Lake Ontario was fantastic.”

For more on What If and other hot new summer projects, pick up the latest double issue of Entertainment Weekly, on stands Friday.