The subverted expectation is one of the major staples of writing techniques. It's how you produce twists at the end (or in the middle) of stories, comedy, horror, and those moments of all-is-lost-oh-wait-we're-saved, or nothing-can-stop-us-now-oh-crap-we're-screwed. There's a long form (story twist) where the reader thinks a particular way for whole chapters, and a short form (comedy, horror) where it's a brief set-up followed by a punch-line. They're slightly different in use and practise, but the basic technique is the same.

The trick is to first lead the reader down one path of thinking - but gently, so they don't realise they're being lead. If you tip your hand by laying it on too thick, the reader can see what's happening - they start looking for the catch. At the same time, they have to be prepared to accept the subversion or inversion when it comes, otherwise you'll jolt them right out the story while they go "huh? What the hell are you doing?" It's a delicate balance.

When the time is right, you pull the rug out from underneath them with a revelation that things are pretty much the opposite of what they seem. It's a trap! They have a cave troll! No, you're a traitorous bastard and I'm going to have your head cut off after all! The rug-pull should be sudden - what you're going for is the lurch they feel when what they thought was true suddenly isn't anymore. That disorientation, the 'ah-ha'. You won't get that if the reader knows what's happening beforehand. If they could have figured it out at any point along the last five pages, you're in trouble.

That's not to say you can't give the rug a few tugs now and then - in fact, in long-form variations it's a necessity, as part of making sure the reader is prepared for the revelation. It's called foreshadowing. But be careful when and how you do. Keep it subtle, keep it a good distance from the real revelation, and keep it in the opposite direction to what's really happening. That is, if someone is going to be revealed as a traitor, don't have them do something treasonous. Instead, have them perhaps quietly filch something, or tattle on someone else's crime. Something that foreshadows - without giving away - the fact that they're a nasty piece of work without showing the specifics.

The actual exercise is short-form subverted expectations. Pick one of either comedy or horror, whichever you feel like writing.

Horror

For horror, start with a situation that's, well, nice. Not boring, but pleasant or exciting. A picnic in a beautiful mansion. A new adorable fluffball of a puppy. Write the opening paragraphs setting this scene. Don't go overboard with the sweetness - remember, you have to lead them gently so they don't see it coming.

About three quarters of the way in, put in an alarm bell. One tiny, mostly-insignificant detail that says "everything is not okay, here". The mansion has an abandoned but warm teapot on the table for no reason. The puppy growls at a cat with a growl too deep for its body. But keep your characters unaware. They explain it away, rationalise it or become distracted by something else. Most of all, don't draw attention to your alarm bell. It needs to be there in the back of the reader's mind so the revelation makes sense, but if you let the reader stare at it too much, they'll see your rug-pull coming.

Write the last quarter, keeping with the happy, pleasant scene and then, in one move (one sentence if possible) turn the whole thing on its head. You need one event to happen that shows what was really happening all along. Don't just have the puppy viciously attack them and them slowly figure out it's a hellhound. Have it morph into Cerberus in front of them. Have the house shrink around them and inprison them in a cupboard. You can't afford to have your characters take a page to figure out what's going on (and therefore tell the reader) - it leeches all the drama from the twist. Remember, the rug pull has to be sudden. That said, it also has to be clear, or you'll frustrate your reader and leave them unsatisfied. There are stories with literary merit where you finish the story not knowing what was going on, but that's not what you're trying to do here. So - be clear, and be concise.

This may mean a lot of rewrites until you find exactly the right way to make that revelation. 

Comedy

Comedy is pretty much an inversion of the above, with one exception - no foreshadowing. You have to have all the ingredients there - you can't introduce anything new for the reveal, but they have to be placed in such a way that the reader doesn't put it together. You can't give anything away with this kind of comedy - it's all about the sudden relief.

Start with something, dark, terrible, scary, whatever. Keep it dark and scary and terrible right up until the last moment, and then make it ridiculous by bringing in something from left-field that makes the scene not-scary anymore. Cthulhu is rising, but discovers he now needs a walkerframe.  It has to be something non-obvious; having the Kraken beaten back by submarine missiles is obvious and not funny. Having it frightened off by a burly fish-and-chipper with a cleaver and a bucket of beer batter isn't obvious. Unless you telegraphed it beforehand. 

Once again, your punchline has to be concise (though that doesn't mean the story has to end there - just keep the moment of revelation as brief as possible) and clear.