Susanna Kaysen's memoir and James Mangold's film Girl, Interrupted both discuss temporary life in a mental institution. The differences between the two are quite distinct if you pay attention.
Kaysen's writing style is a mix between observation and stream of consciousness. She watches the many events going on in her ward while at the same time voices the many thoughts running around in her head. It's in a similar style to Sylvia Plath's The Bell Jar.
Mangold's direction is a trifle different to Kaysen's writing. It appears rather focused on the main character rather than the ensemble of people Kaysen introduces in the book. True, the supporting characters have their time in the spotlight, but only for a short time.
Anyway, onto the differences. As with most adaptations, certain scenes are altered, removed or added. Though some scenes improved in the film, if only a minor tweak or two. In total, only a small number of scenes between the two match.
All in all, both memoir and film have their flaws, yet the memoir comes out on top. Mainly because it's straight from the horse's mouth, not glamorized to make it more appealing to everyone. Then again, the film isn't as sporadic as the memoir.
What's worth checking out?: I'd go with the novel.