Aside from the intentionally disturbing nature of this 1973 film, I find this story about a "possessed" girl to be somewhat confused and only half-baked... Well, what could I expect?
The makers of "The Exorcist" certainly weren't aiming for plausibility here. No. Their goal was to gross the audience out like it had never been grossed out before and make a huge pile of money doing it. I'm sure we'd all agree that they succeeded very well on both counts.
As far as Regan's "possession" goes, I think it was a really screwy one and kinda wimpy, too.
To begin with, since she seemed to be fortified with an incredible amount of physical power, this left me wondering why the hell she would allow herself to be confined, like a prisoner, to her own bedroom. I would think that whatever it was that was possessing her would be itching to go out on the town and, you know, turn a few heads around on Main Street, or something wickedly-fun like that. But none of this kinda thing happened. There were no aborted escape attempts, either.
All-in-all, it was a kind of stupid "demon". Boring and unadventurous, if you ask me. But apparently Regan did get out once (or did she?) to desecrate the virgin Mary icon in the church. And her desecration was so artistically done, too. It was an amazing piece of art work for a mere twelve-year-old whose previous examples of clay figures were lumpy turtles and the likes.