Marion Cotillard and Brad Pitt had the rare opportunity to work on their chemistry for several weeks before starting work on Allied.
The actors appear together in Robert Zemeckis's new Second World War romance thriller as two operatives in Casablanca who fall in love against the odds, marry and have a child together.
Their relationship is threatened when Brad's character Max, a Canadian intelligence officer, is told that his wife Marianne, a French Resistance fighter, may be a German spy.
The actors' on-screen chemistry has been largely praised by critics, and Marion revealed that she and her co-star had an unprecedented amount of time to get to know each other ahead of filming.
Marion told the Press Association: "We had 20 days together with Bob Zemeckis, with Steven Knight the writer and Graham King the producer, who was at the origin of the project.
"We really had this time of preparation altogether, we got to know each other, we got to share our thoughts and love for this movie, it was an amazing moment to get to know each other."
She said it is "rare" to be able to spend so much time before shooting with the cast and crew.
Marion said: "Usually, you can have one reading, one dinner, and then you jump into the shooting of the movie."
She said she "loves preparation time" and the chance to share it with the other actors and director.
"You always have your own time of preparation, but when we all meet there's an energy that's created that really helps to start the movie," she added.
Robert said he had no major concerns over Brad and Marion's potential for romantic tension in the film, but that it is not always guaranteed.
He said: "You cast very talented actors and hope that they have chemistry once they start working together and I could see they had this magical on-screen chemistry.
"I was very relieved and I knew we were in really good shape. But it's something you can never guarantee is going to happen."
Marion, who is pregnant with her second child with partner Guillaume Canet, was previously forced to shoot down rumours that she had any involvement in Brad's impending divorce from Angelina Jolie.
The French star was dragged into their split in September when reports claimed she and Brad had had an affair while working on Allied.
She wrote on Instagram: "Firstly, many years ago, I met the man of my life, father of our son and of the baby we are expecting. He is my love, my best friend, the only one that I need.
"Secondly to those who have indicated that I am devastated, I am very well thank you. This crafted conversation isn't distressing.
"And to all the media and the haters who are quick to pass judgment, I sincerely wish you a swift recovery."
Allied is released in the UK on Friday November 25