In the movie, Blake plays a young pro surfer who, while exploring a secluded beach called Paradise, becomes stranded on a giant rock 200 yards from shore after an enormous great white shark attacks her.
Directed by Jaume Collet-Serra and filmed mostly in New South Wales, Australia, Blake reveals that the scene in which she pulls herself onto a rusty metal buoy and hits her nose wasn't meant to happen.
"I hit my face on the buoy...That really happened, that wasn't in the script," she shared during an interview on Late Night with Seth Meyers on Wednesday night. "And then when I swam up and had this bloody nose, I thought either I'm going to pass out and I'm going to look like a Picasso painting, with a broken nose smeared across my face, or I'm going to have a really badass scene."
While her nose bled profusely, the 28-year-old star thankfully didn't suffer any broken bones.
But she was "thrilled" that the bloody scene was left in the final cut of the movie as she wanted the audience "to know my pain".
"But I'm so glad (the scene was left in), because you know when you get hurt and it hurts a lot worse than it looks, so you're like 'dang it, if you only knew you'd be so impressed,'" she laughed.
Blake, who is pregnant with her and husband Ryan Reynolds' second child, adds that she did many of her own stunts for the movie because the producers kept putting off hiring a stunt double.
"I think they were having budget cuts or something because they didn't hire a stunt double until the last two weeks," she added. Filming for The Shallows began in November, 2015 and Blake was pictured shooting additional scenes for the movie on 12 April,2016 just two days before her second pregnancy was announced.
And the brave star explains filming action-sequences weren't easy, even though the later scenes she shot while carrying her baby were not as physically intense as those captured earlier in the production schedule.
"Doing the underwater sequences, I'm in 4.5-foot waves," she said previously. "Whether I was in the tank or when I was in the ocean, I was about 300 yards away from shore. They would drop me off on this rock that was three feet by three feet."
The Shallows reaches U.S. theatres on Friday and U.K. cinemas on 12 August, 2016.
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