Lisa’s character in the movie is a much more prominent piece of the story than she is in the book. She is mentioned throughout the book, but doesn’t get nearly as much attention as she does in the movie.  Kaysen wrote about her experiences with insight on the lives of the other patients and their histories.  The movie didn’t focus long on the other girls.
More importantly though, are the incredibly ridiculous scenes added in the movie.  The girls are seen sneaking down to the hospital’s basement late at night to go bowling and play pop music.  The hospital was under strict watch, one section of the book was even dedicated to the constant checks done by the staff.  The tight policies of the hospital would’ve made it impossible for the girls to sneak down, especially if they were so loud.  An even bigger plot difference though was the escape scene. Susanna and Lisa plot to escape the hospital so they can move to Disney World and work as Cinderella and Snow White.  They succeed and spend some time at a former patient’s house.  Lisa had attempted to escape many times in the book, but nothing remotely close to that scene ever happened.
Even the depiction of the hospital was inaccurate to what Kaysen wrote about.  The hospital in the movie seems almost campy.  It’s bright and lively.  It almost seems like a joke.  In Kaysen’s version the hospital seems frightening  and unwelcoming.  One of the first people she encountered was a girl painting herself in her own feces.  Grotesque, but accurate scenes like that weren’t in the movie, probably so a more sensitive audience could enjoy it.
The movie was flawed and inaccurate, however it is entertaining. I really liked it when I originally watched it because I didn’t have prior knowledge of what actually happened.  Reading the book really made the movie seem insincere.  The cast did an excellent job portraying the characters, but it should’ve been written in respect to what had actually happened.  The only way to enjoy both is to just think of them as different stories just with the same name.
I think if I was Susanna Kaysen I would be really unhappy with the way it turned out.  She experienced eighteen long horrible months in McLean and to make the movie so inaccurate and to not take her story seriously seems like a slap in the face.  Her time in the hospital burdened her with a stigma of insanity for the rest of her life.  She was wrongly sent to the hospital, and the movie makes all of her trauma seem to be a joke.  It’s just disrespectful.
References
Kaysen, S. 1993. Girl, interrupted. New York: Turtle Bay Books.
Wick, D. Konrad, C.(Producer), & Mangold, D.(Director). (1999). Girl, Interrupted [motion picture]. United States: Columbia Pictures."

(From "http://girlinterruptedm3s13.wordpress.com/", accessed on 26 May 2014)