Wow, I guess this just shows how varied people are. As a person who respects King's abilities but has very little time for most of his (later) novels - I've always thought he was a better short-story writer, myself but then what do I know (what I'm implying is that I'm not a Stephen King fanboy by any means, but I respect him), I found MISERY riveting and, honestly, it's the only book in my life about which I can say "I couldn't put it down" and be telling the truth - I started it one afternoon and spent the whole evening thinking "I have to put this down, I've got to go to work" and then, suddenly, the birds were chirping outside and the sun was coming up and I was done.
Given the fact that you must have some inkling of the general scenario the book eventually reaches (although maybe not) and how limited it is, the highly intensive setting/character building might make a little more sense. You're going to be spending a lot of time with these two people in this one room so there's got to be a lot to draw on in the set-up.
Hope you've read some Somerset Maugham, he gets pretty thoroughly investigated by the end of the book!