The stars play undercover allies who begin to develop a romantic relationship as they plan a high-profile assassination. Pitt plays a British intelligence officer sent to Casablanca on the mission while Cotillard plays a French Resistance fighter who poses as his wife.
And their chemistry was instant.
“Brad and Marion immediately had the utmost respect for each other, so once they delved into their characters, their chemistry was electric,” the producer of the film, Graham King, tells PEOPLE.
The actors spent time together before filming started in order to hone their performances.
“It was crucial for Brad and Marion to not only begin rehearsing together before the shoot, but they also spent a lot of time developing their characters with the director, Robert Zemeckis,” King says about the lead-up to filming. “By the time we got to set, it was really amazing to see these characters already feel lived in and real.”
For more on Pitt and Cotillard, check out the new issue of PEOPLE, on newsstands Friday
Set in 1942, the movie is inspired by real wartime stories heard by screenwriter Steve Knight. It was his script that drew Pitt and Cotillard to the film.
“Everyone was immediately drawn to the project by Steve Knight’s wonderful script,” the producer says. “He is an incredible storyteller and wrote a beautiful love story that examines trust and loyalty.”
Allied – which also stars Lizzy Caplan and Downton Abbey actor Matthew Goode – hits theaters Nov. 23.