There are several things I do, some of which may seem rather strange. . . but I'm a writer and therefore entitled to eccentricity by definition.:P

The first thing that comes to mind is a situation, event or human experience I would like to explore.

Then I invent a character I think will be most suited to the purpose or point of the piece.

After that, I put a fair bit of thought into the necessary factors and events must have made this person into who/what/he/she is. In other words, I map out their history, tailored to the needs of the story I wish to tell in order to mold that character into whoever or whatever I need him/her/it to be.

I find it useful then to have conversations in my head with said character. This is the weird bit.:P I talk to myself (usually not out loud--I'm not quite that far gone). I recognise that the characters I create are inevitably representative of some aspect of my own personality, so it's a useful exercise on several levels. It grants me a greater understanding of my own self while helping to solidify the character into a complete entity, rather than just a fragment of my own psychological being, especially if I integrate knowledge of other people outside of myself. In other words, it's very easy to get to "know" a character if you simply write about real people you know, (you already know them!) using different names, locations and whatnot, but keeping the essense of the person intact as you perceive him.

I hope some of that made sense. If you use a variety of the techniques I mentioned, I'm sure you won't have any trouble generating characters you know like the back of your butt (assuming you're hygenic).

Best of luck!