The 47-year-old filmmaker helmed the new black comedy and McDonagh has claimed he would have been ''f***ed'' if McDormand - who just won a Golden Globe for her performance and is now nominated for a BAFTA - didn't say yes to the role.
Speaking to Time Out London magazine, McDonagh said: ''There isn't anyone else who could play Mildred. There isn't anyone with that same integrity and forcefulness. We'd have been f***ed if she hadn't said yes!''
The film follows the months after the murder of teenager Angela (Kathryn Newton) and there is still no culprit.
As a result, her mother Mildred Hayes (McDormand) makes a bold move by painting three signs leading into the town with controversial messages directed at police chief William Willoughby (Woody Harrelson).
When Willoughby's second-in-command Officer Jason Dixon (Sam Rockwell) gets involved, the battle is only exacerbated.
Although the film has received critical acclaim and has an Oscar buzz surrounding it, the 'In Bruges' filmmaker knew his leading lady would be the best actress of 2017.
He said: ''You don't want to just go under the radar, like 'In Bruges' did in America. We finished this last January and even then I was thinking, 'I can't believe anyone's going to be better than Frances this year.' ''
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