The bathtub novel is the number one atrocity in literature. Okay, so it's mostly a biproduct of great literary fiction, but it does show up in SF/F and, I'm sorry, but it doesn't matter—bathtub novels are bad.
What's a bathtub novel? It's a novel in which the character does NOTHING. They sit in the bathtub, lay in bed, hang in stasis, or wait for the evil sorcerer to come and kill them in the dungeon, but they don't actually DO anything. That's not to say an intellectually based work has no place, but if we're looking at mainstream, commercial fiction (which is sort of where I live, if you haven't yet figured that out), someone has to do something at some point. I think Kurt Vonnegut said it best:
“Make characters want something right away―even if it’s only a glass of water. Characters paralyzed by the meaninglessness of modern life still have to drink water from time to time.”
No matter what your book is about, something happens. Real people do things, eventually. It's inevitable. Even if your character has been chained to a wall and can't escape, he or she is going to struggle and kick and talk and whine and cry and bitch and piss and moan and, assuming they're not starving him to death, eat.
Yes, I'm being preachy here―so sue me―but this point is supported by Vonnegut, and last time I checked he wasn't exactly a total failure. It's not to say characters always have to move around and be active, but they can't be inactive the entire book, or you don't really have a plot. Of course, there are exceptions depending on how the plot is displayed to the reader. Look at Wells' “The Time Machine”. Technically, not a whole hell of a lot happens, but the story is told within the story, and hence something happens in the important part, although it's really only Victorian men sitting around smoking and chattering amongst themselves.
Plus, who knows what's going to come of that midnight trip to the supermarket for pizza? Maybe that's the jumping off point for your story and you'll have to (sit down for this) delete those twenty-thousand words of motionless, introspective, literary brilliance and start with “He wanted pizza.”
Actually, that's a good idea...don't use that line...I call dibs...
Off to write a story about a guy who really wants that damn frozen pizza at midnight,