The Australian actress will take over the role from Rachel Weisz, who was forced to drop out to film additional scenes for The Bourne Legacy and Oz: The Great and Powerful.
Kidman will play a woman who helps her husband try to overcome nightmares caused by his terrible experiences during World War II.
According to the Daily Mail, the actress was persuaded to act in the film by Firth and producer Andy Paterson.
The film is based on the true story of Eric Lomax, a second lieutenant in the Royal Corps of Signals who was captured by Japanese forces.
Kidman, who is married to country singer Keith Urban in real life, will portray Lomax's second wife Patricia Wallace, also known as Patti.
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