All stories have underlying ideas and subtexts which can be expressed in a basic form of theme. Greed. Lust. Death. Love. Power. Corruption. Revenge. Family. War. And so on. Of course, having evidence of one or more of these ideas doesn’t necessarily represent a theme in itself; how the story is resolved through character and plot reveals what you’re saying about the theme (whether you’re aware of it or not).
For example, say your story is about a man who cheats on his wife (suggesting a theme of lust/love). This could be a thriller, horror, romcom, whatever you want. Anyway, in the story, he gets away with his fling but finishes the affair, realising he’s better off with his wife. However, his wife wants a divorce, feeling that their relationship isn’t working anymore. The man tries to resume his relationship with the other woman but is turned down, and is left alone. The theme could be saying that with love, you can’t have everything your own way (but there could be a 100 bloody corpses or a comic wedding for the finale, depending on your genre).
This is a simple example, but you get the idea. People may not pick up on the theme at all, depending on how the story is delivered, and will have just enjoyed the breakdown of the man’s selfish behaviour. And that’s perfectly fine. Maybe even the writer couldn’t care less what the theme is, and is happily leaving it for others to come to their own conclusions. When people mention ‘theme’, it conjures up pseudo-intellectual posturing about deeper meaning when there may be none in the first place. This is what makes theme a difficult subject. Generally, the audience doesn’t want to be aware of it - not consciously anyway - and certainly not while they’re watching the film/TV ep.
The usual comment about theme is that it should be invisible, and this is good advice to follow. By all means, layer your characters and story with your mind-blowing notions of whatever theme you’re after, but don’t bore the audience with dialogue lectures or insignificant scenes that support the theme (and do nothing for the story). However, it’s interesting to note that one of the acclaimed films of the year gets caught up with its theme, and in my view, robs the film of its full entertainment factor. No Country for Old Men (see end of post for my spoiler explanation).
Writing for theme works in a variety of ways. One, don’t worry about it and get your first draft done, only then revising the story to add notions of theme and what you really want to say. Two, write with theme in mind (many writers stick the theme, or even one word, over their computers to remind themselves that everything must feed into that idea) and really get a hold of the resounding power of your story (get you). Three, forget all about theme. Write what you like. If you feel confident that the story delivers what you want in terms of entertainment value but you don’t have a clue what the theme is, don’t worry, someone else will come up with one for you, probably a critic (especially if the film is a hit). Loads of hit films don’t have themes but that doesn’t mean to say that they’re hollow or without merit. The slimmest suggestion of a theme could be enough. It’s a useful reference, and can certainly bolster your characters/story when you’re fully aware of it, but don’t trip yourself up trying to ensure that your script’s got a thematic argument. That way madness (and boredom) lies.
** SPOILERS for No Country for Old Men, do not read if you haven’t seen the film **
No Country for Old Men stands out as a taut and compelling cat-and-mouse thriller about opportunistic Josh Brolin as he goes on the run from assassin Javier Bardem while doleful sheriff Tommy Lee Jones ponders why the world has turned to ruin. There’s a lot to enjoy in the film - the set pieces, the acting, the direction, the pace/silent flow (it’s not a very talky piece) - and then, for the last half hour, the story completely changes direction and focuses on its theme rather than fulfilling its expectations of plot.
The result, a brilliantly flawed bit of cinema. Brilliant up until Josh gets done, seriously flawed after that. Tommy Lee Jones steps forward for the latter part to have a few great ACTING scenes and to talk about the THEME but do little to engage the viewer or explain what exactly has happened. Indeed, even the Coen brothers, in an interview with Creative Screenwriting, have indicated that they just followed the narrative thread from Cormac McCarthy’s book and don’t fully understand the story’s change of direction. Still, it’s profound, so it must be a masterpiece. Be warned, you’ll only receive that kind of recognition when you have the filmography of the Coens and the acting pedigree of Lee Jones et al in your flick first. And don't worry, I love the Coens but I was confused and dissatisfied at the end of NCFOM rather than blown away.
WATCH OUT: there are spoilers in the comments section as well.