the "spider walk" scene in The Exorcist, extended version

Regan MacNeil (Linda Blair, but probably a stunt double in this scene), out for a walk

Here's how the scene is described in the book: 

Chris looked up. Then to the side. And froze. 

Gliding spiderlike, rapidly, close behind Sharon, her body arched backward in a bow with her head almost touching her feet, was Regan, her tongue flicking quickly in and out of her mouth while she sibilantly hissed and moved her head very slightly back and forth like a cobra. 

Staring numbly, Chris said, “Sharon?” Sharon stopped. So did Regan. Sharon turned and saw nothing. And then screamed and jumped away as she felt Regan’s tongue snaking out at her ankle. 

Chris threw a hand to her cheek, her face ashen. “Call that doctor and get him out of bed! Get him now!”

Wherever Sharon moved, Regan would follow.

"Chris" is Chris MacNeil, the mother of little Regan, who gets possessed by a group of demons, consistent with the biblical accounts. Chris is a movie star, separated or divorced from her husband. Sharon is Chris's secretary. At this point in the story, Chris, hovering somewhere between agnosticism and atheism, isn't quite persuaded to go the exorcism route.

I have no idea, but I reckon that the scene was cut from the movie because there was no technology, at the time, to do the scene justice. In the extended version of the film, the scene has been included (different versions of it exist), along with CGI enhancements. It's obvious that they had a stunt actress crawl down the stairs while suspended by cables (which, I assume, got painted out by computer), but since the stunt actress still isn't bent backward quite as far as Regan is described in the book, the scene ends up being disappointing. (Overall, the extended version adds little to nothing to the original theatrical version.)

I rewatched the theatrical cut of The Exorcist recently, while also reading the novel, which I'm currently rereading. Both the film and the novel are worthy of review, each for their own reasons. William Peter Blatty was in his early forties when The Exorcist was published in 1971. The movie was made almost immediately, coming out in 1973.

The only picture I could find that comes close to the book's description is this old photo of a contortionist from all the way back in 1880:

found here

To complete the tableau, the contortionist really needs to have his head closer to his ankles, so in the photo, his body is bent in the wrong place. Also, when I imagine what Blatty meant by "gliding spiderlike," I visualize the contorted person moving smoothly and silently on dextrous fingertips and tiptoes, navigating the staircase in a way that would seem utterly inhuman. With that kind of image in my head, I don't think there's any way that even modern Hollywood could do the scene justice.