The 31-year-old said she was surprised to see that the "Batman" star had no tantrums and was completely focused on his work, reported People magazine.
"He's very smart and very funny and is definitely a practical actor. He comes to work to get his job done and to make the best movie possible but is still really normal-sauce off-camera," Kendrick said.
In the film, the actress plays a clerk, who befriends Affleck, 44, a socially-awkward accountant.
"The idea of the world's sexiest, most dangerous accountant is certainly the kind of delicious dichotomy that makes for a great movie.