That was Shelley Duvall. She comes across as a complete nervous wreck unsure of her own shadow in the movie...

On purpose.

I read something recently where it was said that the director, Stanley Kubrick, badgered her so badly about her performance that she nearly quit. He had her in tears every day. He confessed later that he did it on purpose to get her into the state he wanted her to be in.

Having seen that movie a hundred times it is my opinion that her role was far more than to scream, cry and be a moron. She is truly important to the film. She's a bit mousey and helpless and annoyingly positive, or at least she's trying to be positive in what clearly, for her, is a pretty unfortunate situation, even before Jack goes all the way over the edge.

It is, after all, HER fear we feel. Jack is just so damned good at playing a nut that he steals the show, and rightfully so, but if you look a little deeper it's not even the kid, or even the chef who takes an ax to the chest, that we feel for. It's her. She's not the best actress to be sure, but I have to defend her in The Shining. It isn't the same movie without her.