I was over posting on Absolute Write the other day and a beginning writer ask the question, how do you find out what motivates your characters? I suggested it could be done with something as simple as an interview. I said to interview your character as if you were a newspaper reporter asking probing questions about their life, quest, current situation, and other topics that could yield the answers. Come up with all the questions first. Then conduct the interview. It sounds simplistic, but it works.
As authors, we know how vital it is that all our characters have a goal. They must want something, and that something is what drives them. But it's more than just a want. They must also have a need. If we don't know what our characters wants and needs are, neither will our readers. With nothing to root for, the reader will lose interest. And in the end, they won't care about the outcome.
So what is the difference between want and need? Think of Marlin, Nemo's father in FINDING NEMO. Marlin's only son, Nemo, is captured by a scuba diver and placed inside a fish tank in a dentist's office. Marlin sets out to find Nemo. But he has a big problem, one that's quite unusual for a fish: he has a terrible fear of the open ocean. So with just that much information, we now know his want and need. He wants to find his son, but to do so he needs to overcome his fear of the ocean. The reader (or viewer in this case) will root for Marlin to make it through all the perils he faces in order to find Nemo and rescue him.
Every character must have a want and need. The most critical are the ones for our protagonists and antagonists. But I think that even the smallest, one-time, walk-ons must be motivated. If we determine the goals of every characters, we will have an easier time writing them, and the reader will have a more distinct picture of the character in their minds.
In planning our stories, it's important that we determine our main character's wants and needs first. In doing so, we'll always have a goal to focus on as we write. So what are your main character's wants and needs? Can you express them in one sentence like we did with Marlin? Let's find your Nemo's needs!