If you write fiction, your characters will ask you that at every turn. Of course, I don’t mean they will step off the page and literally put the question to you, unless you are smoking something especially potent.
And I’m not referring to their larger place in the story, unless you are writing short fiction. Every speaking character in every scene must have a dramatic reason for being there. Non-speaking characters, such as those who wander the crowded lobby of a hotel, exist for the same reason as nameless extras in a movie, except that you don’t have to pay them.
When a character opens his mouth and his or her words appear inside double quotes on your page, that person has a reason for being there, for doing what they do, and for saying what they say. While these reasons may never be made explicit in your text, your control over your own story will be strengthened if you know what those reasons are and they drive how you write that character’s actions and lines.
Case in point: I’m currently working on a scene that I felt was getting away from me. There were three characters, and all of them had a perfectly legitimate reason to be in that place at that time. They are FBI agents, and they have a suspect surrounded. What I didn’t have is a solid dramatic grounding that told me how these three people related to each other. As a result, they were just taking turns dumping information on the reader, without actually driving the story forward or interacting with each other in an interesting way.
Thus, I went back and asked myself a few simple questions to clarify this issues in my own head. The answers I came up with may or may not find their way on to the page, but just working this out in my head has shown the “through line” of my scene with a bright laser-like quality.
Suddenly one character is worried that two outsiders are there to usurp his leadership of the scene. The second character is convinced that the first character resents the third, his subordinate and protégé, because she’s a female. The third character (our heroine) doesn’t give a rat’s ass about the politics. She just wants to be the one to slap the cuffs on the bad guy (or more likely, give the order to the sharpshooters to take him out).
Even if I don’t make any of these explicit, having them in my head puts an extra zing behind each sentence each character speak, gives the scene an energy it didn’t have before. Suddenly, they’re people with points-of-view, rather than just a name tag on every third line of dialog.