Characters have always been the easiest part of writing for me – they’re usually the first component of any story to occur to me, often the first to flesh out into something three-dimensional in my mind, and the primary source of conflict in most of my work stems from the characters and/or their own inner conflict(s).
However, I don’t like to limit too many of my plots (especially anything longer than a short story) to being purely character-driven. It’s always harder for me to come up with external conflict I like that’s big enough for a book plot, so I use tools to brainstorm that kind of stuff. One thing I turn to a lot is psychology/sociology. Which sounds like it would lead straight back to internal stuff (which it does, sometimes). But it gets me thinking about the external pressures people face, how different people react to the same circumstances in totally different ways, and how those varying reactions can become another external conflict in the story.
One thing I use to brainstorm is looking over basic human needs. The precise wording, number, and definition varies from theory to theory, but it’s all pretty much the same stuff, just broken down differently. There’s Maslow’s hierarchy of needs, in which the basic needs like sustenance and safety have to be fulfilled before the “higher” needs even matter to a given individual – which is an interesting point, in story terms.
There’s also a Wikipedia article on fundamental human needs with a nice little table with specific goals, actions, qualities, etc. related to each category of needs. It defines fundamental needs as:
How do you make a plot out of fundamental human needs? Well, as usual with writing, be sadistic to your characters. Take some of these things away from them, or at least threaten to. Or make them choose between two. Or set two characters with two different problems with need fulfilment at odds with each other. Or explore a community with an unfilfilled fundamental need, and how individuals’ reactions to the issue affect one another, making things worse for the others or better for the others.
To me, the best book plots often don’t have “bad guys” per se, just people who want different things, going about getting what they want in different ways, pulling at each other or pushing each other away, each one internally conflicted and each one affected by the events around them, as well.