Here's a theory: certain improbable scenarios only support a certain amount of naturalism. And it depends very much on the scenario you pick.

I was reflecting on this the other day as we returned home from the latest Batman movie. It turns out that some people are devoted to Batman, and there's a vast amount of stuff about there about him, mostly comic books; I'd grown up familiar with nothing more than 'na-na-na-na na-na-na-na BATMAN!' somewhere in the back of my mind, but it would seem that there's more to it than that, as witness the increased attempts at seriousness and naturalism in the recent movies.

Now, I like Christopher Nolan's films, and I enjoyed the Tim Burton Batman movies - but the combination of Nolan and Batman really didn't work for me. I suspect that saying this on the Internet is tantamount to singing 'God Save the Queen' in the middle of a Fourth of July celebration, but there it was. Burton's Grand Guignol was fun, and it felt suitable for the subject matter: I was, after all, watching a movie about a man who, for reasons best known to himself, had decided to fight crime wearing the world's strangest hat. An enthusiastically overblown atmosphere worked well for that. In such a bravura world, heck, why not dress up as a bat to fight crime? You'd hardly be more eccentric than anybody else. Nolan's more naturalistic style weirded me out; it called to my attention that, amongst all these rather normal-looking people, there was a man who had decided that the world's strangest hat was an essential component in law enforcement ... and that created a tremendous dissonance in my mind. What on earth was he doing with that hat? Why did nobody blink? Heath Ledger's Joker feeling unable to commit criminal mayhem without extravagant make-up I could just about accept; you'd think you could plant bombs and yell at people without the slap if you really tried, but it didn't strain credibility too far. For one thing, he was supposed to be snooker loopy in a theatrical, performative way: some people enjoy weirding other people out, and he appeared to be one of those, hence it wasn't impossible that such a person would consider a scary white face a useful tool for intimidation. There was a degree of real-world precedent; after all, some people do put on extravagant war paint. It's usually in cultures where everyone does it, but it's a recognisable human behaviour, so the clown-face didn't clang too hard against the naturalism. But, but, but ... nobody dresses up as a bat to fight crime. What were they doing, trying to make such an unrealistic premise look realistic?

But as I heard myself ask that, I realised I sounded utterly inconsistent. I mean, I wrote a book about werewolves that aimed for naturalistic writing. I'd just finished another one about mermaids, and that was trying to be realistic too. Was I a big huge hypocrite, or did I need to reconsider?

So I reconsidered, and in so doing, clarified something in my mind. Because - and this is the general point - there are different kinds of realism. There's physical realism, and there's psychological realism, and they're two separate things.

If you change something physical, then you're more or less pulling off a magic trick. You can rationalise it with science (we've discovered that if you boil protons in strong enough tea, you can make time flow backwards!), you can present it as magic (a woman with spooky powers turned me into a frog!), or you can simply fait accompli your audience (there are zombies in this book because I say so, deal with it or go read something else).

But the thing is, you haven't changed anything essential about people in your world. People are people wherever you go. Shona villagers almost certainly gossip as much as New York bankers; retired French plasterers probably lament about their old bones about as much as retired Indian sitar-makers; Chinese farmers and British industrialists are equally likely to consider their own children better than everyone else's. And if a change in culture, location, socio-economic status or traditions has a limited effect on such things, it's reasonable to assume that they wouldn't be affected by the existence of space stations or witchcraft. They might change people's living conditions, but how people react to those changes would still be pretty human. You have a story about people living in unusual circumstances, but once the initial improbability hurdle is gotten over - and the characters' reactions to their circumstances can help a lot with that - you can be as realistic as you like.

On the other hand, there's the issue of psychological realism - and here we get into different territory. The Batman example is a case in point: people do not, on the whole, react to childhood trauma or law and order difficulties by dressing up as an animal and running around getting into fights. It is a psychologically implausible thing to do. Neither to people generally react to such behaviour by taking it seriously. The Isle of Skye is home to Tom Leppard, for instance, who tattooed himself and fixed his teeth to look like a leopard, but if you watch this YouTube clip of him, you can see that he gets scant respect from strangers; the narrator is more or less openly laughing at him, as are the commenters. His neighbours regard him as a nice guy with some kooky ways, it seems - there's a tradition of great British eccentrics that he fits comfortably into - but he's clearly a shy and uncommon man, doing his own thing in a way that takes him out of society rather than makes him responsible for maintaining it. It's understood by everyone that, while he's welcome to live that way if he pleases, it is a strange choice of lifestyle, and most of the kneejerk reactions you'll see on that piece are some variant of 'What a weirdo'. Those comments are not polite or respectful - it's his own business, after all, and he seems like a perfectly nice man - but that's kind of the point: if you get yourself up as an animal, it does not generally garner respect. If Tom Leppard decided it was time for him to fight crime, especially violently, a lot of people would be extremely uncomfortable. Leppard seems peaceful and charming, but the idea of a man whose relationship to society is so disconnected deciding to arm himself is not a soothing one.

Now, there's no reason not to tell a story that involves some suspension of disbelief about how people behave. Punch and Judy is hardly an in-depth study of behaviour: hangmen very seldom put their heads in the noose to show you how it's done. But it helps a lot that Punch and Judy is performed by squeaky-voiced puppets. They're clearly not real people, so it hardly matters if they don't act like real people; it would be a little unsettling if they did. Caricatured behaviour and caricatured people go together in Punch and Judy, and the result feels comfortably consistent.

If, on the other hand, something in your story demands a degree of suspended disbelief about human behaviour, it throws everything off if you try to make people behave realistically except for that one thing. If they're realistic people, then why aren't they reacting realistically to the one thing; for that matter, why is somebody doing that one thing at all?

It's a question to consider when setting up a scenario of your own. I suspect the main reason why it flew in the recent Batman movies is familiarity: people are sufficiently used to the idea of Batman that it no longer startles the audience. That makes it easier to overlook the fact that none of the characters seem the least bit startled either. But if you're creating your own scenario, you don't have that padding: your audience has not been primed to accept implausibilities. Consequently, any disharmony between the implausibility of the set-up and the characters will stand out sharply.

So, how to balance? If anything, there may be an opposite pull in the two scenarios. If your situation involves some physical implausibilities, having the characters react to them realistically may actually help the audience accept them: one imaginary thing greeted as if it were real, either with the 'But there are no witches, not really,' response of a real person encountering such a thing for the first time, or with the matter-of-factness that people generally display towards the familiar, seems integrated into a 'real' world. If, on the other hand, you have something psychologically implausible, what you need to present is a world in which everyone's psychology is strange enough or simple enough that an improbable piece of behaviour wouldn't seem out of kilter.

A lot of this has to do with imagining the background world. The scenario of Se7en, for instance, is psychologically rather unlikely. People seldom do plan out five murders and a suicide in order to make some kind of religious point; preachers, even extreme ones, are generally less decoratively gruesome in making their points, and serial killers generally kill people because they profit from it or get off on it, and consquently tend not to plan a grand finale after which they'll stop. But the film goes to some trouble to establish so dystopic a background setting - semi-realistic, but drawn in broad, expressionistic strokes - that the killer's motivations seem acceptable, fictionally speaking: he's only acting on opinions that everybody in the movie holds to a greater or lesser degree. His actions are Gothic, but he lives in a Gothic world; it holds together.

If you're creating your own world from scratch, rather than adapting a character you didn't create, harmony of tone seems more likely. You will, after all, be creating both the scenario and the characters; coming from the same mind, they're more likely to mesh. But as a general point to consider, where you need to put the plausibility in your story, what degree of realism and unrealism you can get away with, is worth considering when you're working it out.