Many writers have an image in their mind of how to begin a plot. First, you come up with one or more compelling characters who want something badly. Then you make it hard for them to get it.
Over time, I’ve heard others talk about how they like to envision the situation that starts their story. One said he sends his characters up a tree and throws rocks at them until they find a way to come down. I heard Michael Palmer, the writer of medical thrillers, say he imagines a cannibal’s cauldron, puts his characters into it, lights the fire and nails down a cover on top of them. Then he watches them figure a way out.
Sol Stein, one of the great editors of all time, in his book, Stein on Writing, talks about the crucible, which he describes as “the container that holds the characters together as things heat up.” According to Stein, “characters caught in a crucible won’t declare a truce and quit. They’re in it till the end.”
I write dystopian fiction and think about it a bit differently. I like to create characters with an intense aversion to the dark, place them in the darkest place I can imagine and watch them fight their way back to the light.