Writing, the cliche goes, is 1% inspiration, 99% perspiration. I'll leave aside the insult to people who actually do sweat for a living (let's face it the only time we writers crack a bead is waiting to go into a pitch meeting, checking our bank accounts or lugging two reams of A4 paper back from Rymans) and explain why this is a dangerous concept to embrace.
The 'perspiration' of the adage of course refers to the simple act of filling pages, and while it's evident that you don't get anywhere as a writer by sitting around having ideas, the 1% tage does inspiration a great disservice. The mistake made is that people assume that inspiration is some nebulous thing that strikes like benevolent lightning, once in a blue moon and that's nonsense.
Inspiration, as its etymology suggests, is everywhere and we're breathing it in all the time. Legend has it that Dan O'Bannon got the idea for Alien while casually reading a book on entomology (see, I know the difference) and came across some micro-organism that had exactly the same physiology and life cycle as his magnificent creature. The Sweeney was devised by three writers on a fearsome drunk with no taxi fare who spent the night in a Soho all night cinema watching The French Connection over and over.
These might seem like one off moments of serendipity, but the truth is we're all constantly imbibing inspiration like that. Whether it's in the speech patterns of the guy who sits on the bus talking to anyone who'll listen, who's timbre and rhythms end up as the voice of a minor character in your script, or the documentary you're watching about the history of the A-Z (I got a whole episode with a killer pay-off line, out of that one). It might be the eerie juxtaposition of two incongruous objects like the thigh high, glass stiletto fetish boot mysteriously sitting by a dead crow in my back yard right now, or something as minor as the subtle shift in body language of a barmaid when she actually fancies the thousandth customer to hit on her.
The point I'm making is if you wait for inspiration, you'll be on your arse for a long time. Realise that every second, including sleep, is inspiration, you just have to identify it. Train your senses to take everything in as dramatic or literary forensics , make the actual effort to write down those casual lines and mannerisms you see when you're out and the facts you learn from books and TV when you're in. Build up a massive and encylopedic reference section both in your memory and in your notebooks; the more solid and multi-layered it is, the more realistic your writing will become.