Creating characters is probably my favourite part of writing (apart from finishing and drinking celebratory wine, that is), but the best ones seem to just show up and start talking. To be quite honest, I don't like to think too hard about how they're created, which leaves me in a bit of a bind when they don't obligingly spring fully formed from my subconscious.
Luckily, there are a few methods that work really well for creating new characters (or developing existing ones). The one that works best for me is to open a blank document, separate to my WIP (work in progress) and to just free-write everything I know about the character. Just the act of typing while focusing on the character often releases all kinds of details I didn't realise before.
If this doesn't work, then you could try writing a conversation between your character and another person from the story. There's no pressure to write well or to construct something useful for the book as it's not going to be used (although you may end up with the seeds of a new 'real' scene). Alternatively, you can 'interview' your character by asking basic questions (in writing) and just 'listen' to their answers, or drop them into a random dramatic situation (being fired from a job, splitting up with a partner, doing a skydive) and see how they react. I realise this all sounds a bit mad, but it can be surprisingly productive.
Another handy tip is to 'cast' your character. Look through magazines/the internet for pictures of people and pick one (or more) to represent your character. Having a clear picture of the way they look can spark inspiration.
A popular method recommended on writing courses and in advice books is the character sheet or profile. This is a list of questions (things like height, hair colour, nationality, hobbies and favourite food) that you fill in for your character.
If you are less alarmed (and, I'll be honest, bored) than me at the prospect of a character profile, there's a template that you can try here. Also, here's an interesting piece from an author trying out character sheets for the first time.
One definite benefit of character sheets is the way in which they force you to create your character in detail. Specific and interesting details make compelling and memorable characters. Don't stop at 'they like films'; decide what kind of films they like (and why). A character who likes reading is generic, a character who reads nothing but the works of Andy McNab or Tolstoy is specific. A character who pretends to read nothing but Tolstoy while actually consuming Mills and Boon is even better.
Also, it's important to give your characters a mix of positive and negative traits. There is nothing realistic about an entirely good heroine or an entirely terrible 'baddie', and some of the best characters walk the line between so that you're never quite sure how they're going to act.
Something that sounds obvious, but is easy to skimp on, is the importance of seeing every character as a fully-rounded person. Think about it; the minor characters in your book don't know they're minor characters. If you create them with as much care as you do your major characters then they will not only be memorable and distinct for your reader, but they might help write your story, too. By thinking about their personalities, hopes and desires, then you may find your minor characters begin to act in ways which develop your plot.