Wired has a great interview with Joss Whedon. It's very long, but a great read if you're a fan of his, or interested in his thoughts on writing, characters, and plot. Here's the part that I want to talk about though, it's about characters and their motivations:
Reading this made me feel good because it's something I've always tried to do with both my characters and my plot. I think it's important that in any scene you write, you should be able to turn to each character there and ask "Why are you here?" and they should have an answer. Whether the reason is personal, "I'm here because I love him." or not, "This is my English class, I have to be here." they should be there for some reason that has to do with THEM, and not your plot. If I ask and my character answers, "I'm here because you need me to overhear this argument so that later I can use that info to solve the mystery." then, in my opinion, I've failed to make him three-dimensional, he's merely a plot device in the shape of a person.
Every character, whether they're the main character or one who pops in for one scene, should have a full life, regardless of how much we see of it. When people appear only to prove a point, or drop a clue, or to tell us something about the main character, the whole world of your story feels a little less real.
Achieving this can be tricky. You don't want a minor character to walk into a scene and say, "I'm here because this is my English class, where I'm supposed to be, and I just noticed that your hair looks different." Subtlety is key. This is one of those things where the reason doesn't always have to be spelled out on the page, but YOU need to know it. When you know why a character is there, it shows in your writing, and scenes feel more real.
When it comes to plot points, I always check that all the characters involved are there for a reason, and not because I NEED them to be there in order for the story to move forward. Without that reason -- personal or practical, things can feel "too convenient" or false. You want those moments to feel inevitable, where your readers can almost see it coming, as they weave all the pieces together, and they think, oh no!, at the same time that they think, of course they would all end up in this place just as the bomb goes off, it couldn't be any other way.
Because that's the moment that really connects with the reader. That's where the emotional connection to the story comes in. When they can look back at everything each character has done, and know that this is exactly the way it has to be, because they understand why each character has done what they've done so far, and why they're there at that moment. Without that it's just another thing moving the plot along.