The person at the top of the character list, the one we think about first when someone inquires of our story—he is the protagonist. Almost all facets of the story will relate to him in some way. He's called the lead character because he's going to lead the way through the story. He does things; he makes things happen. He seeks and finds, seeks and stumbles, seeks and finds, seeks and loses. Figuratively, he is the force, the underpinning, the engine, the yeast, the seed, the nucleus, the sun. Literally, he is the reason readers turn the page. 

Without him, we have nothing.

At all times, we have to know what our protagonist wants, and which obstacles prevent him from attaining it. If we don't know, or if the answers are too weak to sustain the story, we need to stop and reevaluate this character. Is he really the one qualified to tell the story? 

Let's say the lead character's conflict, aptly introduced in the first chapter, is to save his girlfriend from a dangerous situation. She's in another part of the world, a hostile war-torn country, and he has to hop a plane to go rescue her. Starts off great, right? Readers will hang around because they want to find out whether he will save his girlfriend.

But let's say that in the very next chapter, he arrives in the foreign land to find his girlfriend safe and sound. Tension is released. Conflict is resolved. Now what? We've already set up the premise that this story is about a man's search for his girlfriend, yet we've settled that pretty fast. It's no longer an issue. This is where readers scratch their heads. They aren't sure where they're going anymore, but, judging by the weight of the book, they presume there has to be more, a lot more, to this story. So the readers read on. 

A few more pages, and we learn the girlfriend has come to this foreign land for a specific reason: to document a stunning scientific breakthrough. The focus shifts. She, not the protag, is the one who speaks the language. She, not the protag, is the one who knows her way around the culture. She, not the protag, can make a difference as a direct result of her scientific expertise. 

Suddenly our leading man has little to do. After saving the girl, he's relegated to aimless pursuits: holding up walls, doling out pensive looks, cutting loose a quip here and there and generally taking up space. He becomes downright pitiful to watch, and readers begin to wonder why the heck they're following him around in the first place. 

Clearly, we have chosen the wrong protagonist. 

So we're left with two options: give the story to the girlfriend or give the expertise to the man. The first one provides us with a new protag. The second one provides us with a new conflict. Either way, the lead character is leading again—our qualified storyteller. 

IN A NUTSHELL
Know who's telling your story.