struggled to gain weight for her new movie 'Conviction'.
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.
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 gain weight 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."