In this post, temporus
asked about a trope I've seen often in historical romances (as opposed to historical novels that have romance). You should read his post, but briefly, hero and heroine see one another, even in dire circumstances, and are immediately hot to trot.
I don't want to slam romances. There's enough of that going on already. This particular trope appears to be popular in romance. I find I seldom can get into novels where it happens--there's a popular book, I keep forgetting the title, something about spies, that opens with the h/h in a French prison during the Revolution under threat of death by an evil torturer, yet they encounter one another and whammo! Instant hots. I put the book down because my willing suspension of disbelief had just fizzled out.
Not because Romance is Bad, or the author was incompetent, it was that I no longer believed in the danger if the protagonists had the luxury to fall instantly in lust and only be able to think about each other. The phrases, the reactions, made it clear that they were destined for a bash on the Beautyrest, which meant their escape was a foregone conclusion, which meant the danger wasn't real . . . which meant no story tension.
A zillion other readers disagreed, as the book is very popular. So after temporus
's post, I was thinking about levels of suspension of disbelief. How much realism do we really want? Maybe that can't be defined in any useful sense, maybe it is more of a balance of many things that have to work together, but even then I doubt there is any sure formula.
Romance readers want the protag to get to intimate space ASAP and stay there. That means most of a novel's focus is solely on them, their sensories, their emotions. I find I can only get into that if there is a huge dose of the funny. So if the above novel had snapped with wit, I probably would have stayed right with it. Therefore I'm inconsistent, unless I look at the question as a combination of tropes and other ingredients.
I am very willing to sink into the world of a novel or a film--I still, at my old age, have problems with certain films because I have to make a real effort to pull myself out if the anxiety level gets too high, the humiliation is too acute, or of course horror too intense. As a kid, I couldn't pull myself out of either film or book. I recollect when our babysitter insisted on watching monster movies, and I was too terrified to go in my room alone (because of course the cheesy five cent monsters on the tube were in my room waiting for me if I ventured in there without my little sister's presence), it was the commercials that broke the horrible spell. I remember the sweet relief as each came on, and then the sick terror when the show returned and I had to endure another fifteen thousand hours, measured in minutes. Whereas I hated commercials if the show was something I wanted to watch.
However I can break that fourth wall in film now, with concentration, but in my reading, I find, it breaks much more easily on its own. Even repeated misspellings, or phrases, can break me out of a reading experience. I hate that. Just as I couldn't break myself out by will as a kid, I have trouble willing myself back in as an old bat. There has to be some compelling aspect to pull me back, and I can't always predict what it will be.