The Oscar-winning actress had to put on 15lbs for her role in the film, which is based on a true story, and admits she had to completely overhaul her healthy lifestyle to achieve the feat.
The brunette star - who portrays Betty Anne Waters, a single mother who, despite never finishing high school, put herself through law school whilst working as a waitress to clear her brother Kenneth on a murder charge - said: "I personally have difficulty keeping weight on, and for this role I had to add 15lbs. My character, a single mom, hadn`t any opportunity to stay fit. For my own needs, I like working out. But to gainweight I stopped that. Besides, this was an independent film. Small budget. We filmed long hours, long days. No time to work out.
"Normally I only eat until I`m full. On this shoot, I kept eating more. Even when I was full, I`d eat more. With nowhere to go, the weight stays on you."
Although she had to drastically change her body and appearance for the role, Hilary doesn`t worry about what she looks like on the big screen because she does whatever is necessary for a movie.
She added in an interview with New York Post newspaper columnist Cindy Adams: "Fortunately, I needn`t worry how badly I look in a movie. I always play characters so it doesn`t matter."
As well as piling on the pounds, the 36-year-old Hollywood star also had to learn to speak with a Massachusetts accent, something she also found difficult.
She said: "I needed to develop a Massachusetts accent, which I worked on with a dialect coach for months. That, too, was hard-won."