I generate a lot of my story ideas just by taking a random situation and teasing it apart logically - exploring different ways in which it could have come about, and different places that it might lead to.
A favourite example is this (which I had considered using as the opening scene for my surreal comedy film, The Catalyst [I wasn't actually going to call it that, but it was a convenient working title, since it summed up the core concept], which I wrote about a little earlier today): a man is making breakfast in a strange apartment, is uncertain if the milk is out of date, so gingerly takes a sniff of it before adding it to his coffee or cornflakes. It's a scenario familiar to most of us. I think the idea of using something like this in a film was initially suggested to me by William Goldman's fascinating book on screenwriting, Adventures In The Screen Trade, in which he observes that such simple moments of everyday human fallibility can be an effective means of establishing audience identification with a protagonist - he gives the example of a gumshoe movie (I think it might have been Paul Newman's Harper) in which we see the hero in the opening scene being forced to reuse a coffee filter salvaged from the bin.
But I thought to myself, How far can we take this, how many times can we reiterate the gag? What if it's not just one carton of milk that's gone off, but two, three, four... twenty?
Once we find that the fridge is full of part-used milk cartons, it gets a bit surreal. And it presents us with a mystery - WHY would anyone have this much milk?
I found an answer to this - one that sets up an intriguing back story, and creates tension, threat for the protagonist.
Why is he in a strange apartment? Because he's just slept with a girl for the first time. This immediately adds to the potential stress and embarrassment of the situation. Why doesn't she have any fresh milk? Is she just a disorganised slob? Well, he's not going to want to complain about that, is he?
Why does she have so much milk? Well, who drinks a lot of milk? Athletes... body-builders. So, her ex-boyfriend is a body-builder, and he's still obsessed with her, has become a stalker; he's in denial about the break-up, and is bullying the milkman into continuing to deliver his order of several pints of milk a day to the apartment, in anticipation of an imminent reconciliation. Hence, not only does our hero find himself in an apartment mysteriously awash with milk, but this leads him to the discovery that his new girlfriend has a complicated past, and that he may be under threat from a burly and psychotic ex-lover of hers.
[This scenario was, I think, suggested at least in part by Roger Doesn't Live Here Any More, a superb BBC2 sitcom from about 1980, written by John Fortune - there was only ever one series of it, alas. One of the gags in this was that the protagonist, going through an acrimonious divorce, had found himself a very beautiful but rather kooky girlfriend, who was also intermittently dating a professional wrestler. She later conceived the idea that perhaps the illicit thrill of adultery was a crucial element of their attraction, so when Roger's divorce finally came through, she immediately went out and married the wrestler, to maintain the excitement in their affair.]