The 58-year-old filmmaker and screenwriter - who helmed the action/thriller 'Nightcrawler' - recently filmed the new crime/thriller but admitted the film nearly didn't go ahead unless Washington, 62, joined the movie as Gilroy specifically wrote the part for him.
Speaking to Business Insider, Gilroy said: ''I've never written so specifically for an actor that if they passed on it I wouldn't have done it.
''I always had a list of people I would have followed up with. This one I did not. I felt very strongly that the character is somebody who believes deeply in things, he's someone who believes there's something bigger than himself. Him welding to that character was a quality I wouldn't be able to find in another actor. I felt very strongly about that. We sat down to have lunch and an hour into it he stuck out his hand and said 'let's do this movie together'.''
The new film follows Roman J. Israel, Esq (Washington) whose life is upended when his mentor, a civil rights icon, dies.
He is recruited to join a firm led by one of the legendary man's former students - an ambitious lawyer George Pierce (Colin Farrell) - and begins a friendship with an equal rights campaigner (Carman Ejogo).
But through a number of turbulent series of events, the activism which has defined his career will be put to the test.
Gilroy also admitted he even let the 'Fences' actor join him in the editing room.
He said: ''I couldn't have really conceived before this of letting an actor come into the cutting room. Most actors are not objective.
''But I knew I wanted him to come in and look at the character and in the process we started asking each other, do we need this scene? Should we trim this? Egos really got checked at the door.''
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