As I've mentioned, I've learned a lot about writing from the film and television medium. One of the most helpful sources of information in my writing journey has been DVD commentaries. Now, some are pretty worthless when it comes to learning anything; they are meant more as entertainment than education. Some, though, are wonderful.
A prime example of this type is the commentary that writer/show runner Ronald D. Moore contributed to each episode of the recent Battlestar Galactica series. Although the show was considered to be science fiction, budget necessitated that each episode focused on the cast, rather than computer generated effects. Personally, I believe that was the show's success. To continue, Moore stated that he was having trouble writing the final episode; he went through several drafts before finally ditching the script and starting over. Though there was a complex series of subplots that needed to be wrapped up, Moore realized what the viewers wanted in the finale. "It's about the character, stupid." He wrote the phrase on the black board in the writer's room, and they all felt focused enough to send the show off on a high note.
This concept shouldn't be a surprise to anyone. If you think back to all of the books you've read, or good films you've seen, the ones that stick with you are probably the ones who had characters that resonated with you. A memorable plot is a terrific first step, but if it doesn't involve interesting people, then you really haven't written a whole piece.
To use another film example, there have been twelve or so Friday the 13th films. Each of them involves a masked killer discarding a group of teenagers in various fashions. Most of those films are watched once or twice, and then completely forgotten. The Final Chapter, as the fourth one was titled, is one of the only ones that stands out. It's got the same plot as the rest. (The villain dying is a staple of all of them, so that wasn't the draw.) It's got the blood, the guts, and the effects that the others have. The thing that really raises the quality of the film is that it has characters that you would watch outside of a Friday movie. The writer described it as a John Hughes movie with Jason Voorhees.
The point is that the people you are writing about are more important than the situations they are in. As readers, we want depth to our story. That depth can really come about by showing changes in characters we care about. There are hundreds of shows/movies about people flying in space ships. Why did Battlestar succeed while the rest were canceled? It was because the characters took the forefront. They weren't perfect people, by any stretch. They were flawed. Over the four years of the series, they grew and changed. Some became better people, others took a fall. Viewers didn't tune in each week to watch CGI robots fight; they wanted to see the people.
How do you make a character interesting? Remember all those notes you've been taking? That's how. By watching the people around you in your daily life, you can find the idiosyncrasies that make a character come to life. Talk to people that are different from you. Find people with odd jobs, or strange hobbies. Visit places that you aren't immediately comfortable in, and see who else is there.
Many times, you can jump start a story idea by finding the right character. In a short story I wrote, Dinner for One, I used a woman that I had seen at a local restaurant. As I waited on my food, I saw a woman sitting by herself. She had a sad quality to her, a vulnerability that stood out to me. I was immediately interested in who she was, and how she got to this point in her life. Of course, you will rarely get the chance to really get this information. As a writer, though, you are free to imagine the details and mold them to fit your needs. In the end, the woman became a secondary character in my story. I focused on a man who noticed her, and developed a sense of empathy because of it.
Before starting a writing project, take a few minutes to flip through old books that you've enjoyed. Take a look at the characters, and see what it was that you liked about each of them. Did they undergo a radical change? Did they have a physical quality that intrigued you? Was it how they spoke? Did they have a different way of looking at the world?
Now, take those ideas you've got and throw in someone you saw at the supermarket, or in a bar. Why are they there? Who will they interact with? See, you've already got something going!