"I went to DC and I met with 11 lobbyists, mostly women. I wanted to know what they struggle with and how they became a success in that environment.
"I talked to them about their families and their personal lives. We discussed what it really takes to get a bill passed and the different ways you would go about it. I learned what grass roots and grass tops were and how the various strategies of running a campaign work," Chastain, 39, said about her role in the John Madden-directed movie.
"He's so generous on set. Even if someone comes in for one day, John does anything he can to make that person feel a part of the team. That's why people like Christine Baranski and Jake Lacy came in for just a short time. Each day I saw the actors I would be working with and knew I was going to be in the best hands, because John had orchestrated the whole thing."
The actress says she was attracted to the role because it told the story of an "intelligent and ambitious" woman. The movie releases in India this Friday.