Sally Field had to fight to get her role in Steven Spielberg's new film, Lincoln -- and then she had to pack on the pounds to keep it!

To portray Mary Todd Lincoln, the wife of Abraham Lincoln (Daniel Day-Lewis), in the historical drama, the 66-year-old actress needed to gain 25 pounds.

"It was hard, it was very hard," Field told Jay Leno** Thursday on The Tonight Show With Jay Leno. "It wasn't as much fun as you would think." She joked, "First of all, it was horrifying because I'm of a certain age and I kept thinking, 'If I don't do it right I will drop dead halfway through this and someone else will get the role!"

So Field attempted to gain the weight strategically.

"I tried to do it healthy, which I understand there isn't any way to do it healthy," she said. "I ate brown rice and these protein drinks made out of stuff called pro-Gain . . . Oh God, it was just disgusting. I never had a fun meal, basically. It just was discipline."

Unfortunately, losing the weight after filming was even harder for the actress.

"I stopped eating, and I'm hungry now," Field joked. "It took me about six months to gain it, and about a year to lose it. I began to work out like a mad fiend. I had two trainers. I really worked out hard 4 times a week," the actress explained. "I eventually had to have knee surgery. My little frame couldn't hold this amount of weight."

This article originally appeared on Usmagazine.com: Sally Field: I Lost and Gained 25 Pounds While Making Lincoln