Blake Lively Dishes on Intense Shoot for 'The Shallows' : 'It Was Like Training for a Marathon'

Blake Lively chats with PEOPLE and Entertainment Weekly editorial director Jess Cagle about her physically grueling work on the thriller

Blake Lively has faced off against Manhattan It-girls, superpowered villains and a curse of agelessness, but in The Shallows, she found herself eye-to-eye with an even more difficult costar: a seagull.

In the new thriller, which sees Lively’s character stranded just off-shore after a brutal shark attack, the actress is alone for nearly every scene in the film, save for her winged partner.

“He was my friend, and he also terrorized me,” Lively said of the bird in the latest episode of The Jess Cagle Interview. “I was doing scenes where I’d be screaming and crying … and suddenly I’d feel a little pecking at my leg. It became a Hitchcock movie. Then we became buddies and he would hop on me.”

As Lively told PEOPLE and Entertainment Weekly editorial director Jess Cagle, much of The Shallows was filmed in a wave pool, and the seagull would latch on to her when the water got too choppy.

“I was like Captain Morgan sitting out there in the middle of this pool with this seagull friend,” she joked. “But he was cool. He was my buddy. He was super sassy. There can be an entire film of just our outtakes of us talking to each other. I thought the movie was going to be a comedy by the time we were done shooting it because of this crazy bird.”

For more from Blake Lively, go to people.com/jesscagle and tune in to EW Radio (SiriusXM ch. 105) all weekend

Directed by Jaume Collet-Serra, however, The Shallows is far from a laugh riot. “They cut all that stuff out,” Lively said of the gull interactions. “I was like, ‘What happened to the bird stuff?’ They were like, ‘This is so a different movie. Not this movie.’ ”

Beyond her unpredictable costar, The Shallows also presented Lively with a great physical challenge.

VIDEO: Blake Lively on The Shallows, Having Fun with Fashion and Her Amazing Burger Hack

“I had to become an athlete,” she said. “It was like training for a marathon. When we were shooting, we were shooting 13-hour days, no lunch breaks, in the water the whole time, 4-foot waves. Just to stay present was hard, and then acting and acting under water – that was really challenging.”

Watch Lively’s full interview above.

The Shallows surfs into theaters on Friday.