"Once we got the scene down, we were told to improv," Mendes, 36, told PEOPLE at the movie's Manhattan premiere Monday. "We got into a little tug of war, and I picked up what was supposed to be a prop knife."
Unbeknownst to the actress, "it was actually a carving knife from our meal. I lunged at him, and I was really proud of myself."
Then she realized her mistake.
"When we cut, Will said, 'Wow, usually we have to coordinate that with the stunt department.' It could have gotten ugly in a second," she recalls. "I almost killed Will Ferrell!"
She also succeeded in nauseating him on set, thanks to a strategic use of undergarments.
Mendes plays the inexplicably hot wife of Ferrell's dopey character in the film, which also stars Mark Wahlberg. But for one scene, she ratchets down her sex appeal. Way down. Dressed as her grandmother, Mama Ramos, she pays a steamy visit to her husband on the sly.
"It was hilarious to see Eva in granny panties and a gigantic grandmother bra," Ferrell, 43, told PEOPLE. "It's almost hot ... and then you get nauseous. So it's both ends of the spectrum."