Other actresses such as Rosario Dawson, Jessica Biel, Malin Akerman and most recently Keira Knightley were rumoured vying for the role, but McAdams, 35, has been the frontrunner for the past few weeks, reported Ace Showbiz.
Scheduling was the only main issue since the 'Sherlock Holmes' star was about to film "Spotlight", but a deal has reportedly been worked out where she could do both projects.
Meanwhile, Taylor Kitsch is said close to landing the third male lead. He is up for the role of Paul Woodrugh, a handsome, 28-year-old military veteran who has seen his own share of violence and destruction.