The 'Mean Girls' actress - who has been looking worryingly thin since her break-up from girlfriend Samantha Ronson - was told by producers of 'The Other Side' to pile on the pounds because the part requires it.
A source told Star magazine: "Producers want Lindsay looking like a normal, healthy girl, not a skeleton. So they ordered her to rest up and eat more. They refuse to risk her dropping out of the movie because of her problems."
Lindsay landed the role in the comedy earlier this month, her first big-screen outing since 2007's flop 'I Know Who Killed Me'.
Commenting on her casting, the film's director David Michaels said: "Lindsay's very aware of people's perceptions of her. She's really committed to doing what she has to in order to change that."
The 22-year-old actress recently complained the attention her personal life attracts is harming her career.
She said: "When I was a kid, I thought movie stars were people who wanted to act in great movies. It's all I ever wanted to do, but I've become this girl who's seen as someone who parties, who wants to be photographed all the time, isn't focused and can't be on time. That's not true."