The two actors star in upcoming comedy "The Other Guys" and one scene sees Mendes lunge at the funnyman with a blade during a heated argument.
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The actress admits they improvised much of the scene, and when she picked up a carving knife, she didn't realize the object was real.
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She tells People.com, "Once we got the scene down, we were told to improv. We got into a little tug-of-war, and I picked up what was supposed to be a prop knife. It was actually a carving knife from our meal. I lunged at him, and I was really proud of myself," she said, presumably meaning before she realized her mistake.
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"When we cut, Will said, 'Wow, usually we have to coordinate that with the stunt department.' It could have gotten ugly in a second. I almost killed Will Ferrell!"