Producers have announced plans for a remake of the crime movie with Michael Hoffman directing from a script written by brothers Joel and Ethan Coen.
The film follows the story of an art curator who attempts to con one wealthy collector into buying a Monet painting that is actually fake.
The 1966 film saw Sir Michael Caine and Shirley MacLaine in the lead roles and it was nominated for three Oscars. Diaz will play a Texan rodeo worker that is in on the scheme.
Speaking to the BBC about the role, Firth said: 'It's a lovely, witty Coen brothers script. I'm told it's a vast departure from the other film.'