The duo appear as Macbeth and Lady Macbeth in the Justin Kurzel-directed version of the famous William Shakespeare play.
Speaking to the Daily Mail, Fassbender explained the inner turmoil his character experiences over the course of the movie.
He said: "He's suffering from post-traumatic stress disorder. It makes total sense, when you think about it. Justin set the seed of the idea in my head.
"This trauma is something we know about. In World War I they called it battle fatigue, and it was probably more horrific in Macbeth's days, when they were killing with their bare hands, and driving a blade through bodies.
"He's having these hallucinations, and he needs to return to the violence to find some sort of clarity, or peace."
The film will retain the original text's setting, taking place in war-torn 11th-century Scotland.
Producer Iain Canning added that all the cast have adopted Scottish accents in the film, apart from Cotillard, who will speak in her natural French accent.
He said: "We felt it would be unreasonable for her to put on a Scottish accent. It would not be unreasonable to presume that her character spent time in the French court."
Natalie Portman was originally set to play Lady Macbeth in the adaptation, but was replaced by Cotillard last August.
Macbeth will be released in early 2015.