Okay, I'll get us started... The protagonist is what makes your story unique. The same thing happens to two different protagonists, you have two totally different stories. So to create a nuanced, original and engaging story, you need a protagonist with those qualities.

That's a good point, Debbie, one that's so fundamental, it's likely we might overlook it. That story is the Protagonist's story, completely unique to them. It can only be their story as it is tied to their personal history, where they are in space and time when the story starts, and what their destiny is. So Protagonists are important because they are unique and bring that uniqueness to a story. What else?

It's funny because it almost seems like a trick question. Debbie is right on in her reply about the protagonist creating a specific and unique story even in the midst of similar setups. I also think a protagonist is the audience's way into a story. Sometimes that might be as an "everyman" who we can relate to, or as a "hero" type we aspire to be, or even just as a conflicted jerk who we're interested in seeing because he(or she!) reveals something about ourselves in their reaction to events. Too, the element of change is something we seem to be very tied to. It doesn't seem purely cultural, as catharsis has been an important part of drama for a long while, but we like seeing someone move from point A to point B (for better or worse) - that's can be the crucial journey for the protagonist that the audience is drawn to. Even if there is no change for the character himself, perhaps how the character changes the world could also be important (I'm thinking about "hero" types here.)

Change is a big thing for protagonists, sure. I suppose you could also say the protagonist is the audience's way into the theme of the story, too. So Inception isn't just a story about "letting go of your past" - it's a story about a specific man who has to let go of his dead wife or he'll literally spend forever trapped in a fantasy world. The protagonist is living the theme, embodying the theme.

The protagonist is the story. The only reason there's a story is because there's a character who wants something -- wants to change something, wants to become ... wants, wants, wants. Without that want there is neither a story nor a protagonist of the story. The story emerges because there is a gap between what the protagonist wants and what he/she needs in order to obtain, become ... etc. What the protagonist wants is something that we (audience/respondent) can identify with. What the protagonist needs is something that we can see, but the protagonist can't. We watch because the dramatic tension increases as the protagonist draws near to getting both aligned, and then we watch them scatter asunder, and wonder how he/she is going to get back on track. What the protagonist wants is a life and death desire, what the protagonist needs is the internal wound/flaw/lack that keeps the desire at bay. Managing the gap is the key to the dramatic tension. I consider a story to be like a triangle with the Protagonist at the top (the reason there is a story), the circuitry of other characters at the second point and the circumstances they all find themselves in at the third. These points though, are not what we (audience/respondents) are interested in. We are interested in the lines that connect the points: the relationships between the Protagonist and the circuitry of other characters (all of whom have a relationship with the protagonist, else they have no business being there), the situation which the Protagonist finds him/herself in (which occurs in the space between Protagonist and circumstances), and the movements and effects (gadding) that occur between the circuitry of other characters and the circumstances. Movements and effects are the dynamics of how other characters affect the circumstances, and how changed circumstances affect the circuitry of characters. The Protagonist is the character with whom the audience is expected to identify (develop sympathy and empathy), and demonstrate to the audience the proof of the premise. It is the protagonist who provides the point of view in terms of the ideological platform of the story (even though they may not necessarily be the narrating character), the social and aesthetic relationships between the characters and their story world, and provides the means for the writer to offer up a view of the cultural relationship he/she has with the audience. Understanding the motivations, goals, behaviours and judgement of the protagonist is the key to understanding the dynamics of the conflicts playing out in the story. We are deeply interested in the conflict and how it will be resolved and the more the audience is able to engage in the protagonist's actions and decisions, the more engaged they will be in the story. On the other hand, if the protagonist is shallow, without conviction, without a clear objective, without powerful motives, the conflicts are likely to have little depth and meaning, and the audience will have nothing to act as a handle for the story and its premise. Really understanding what drives the protagonist is the key to creating story.

Protagonist? Causal variable. THE causal variable. The sun in the solar system. And our door into the story. Who we attach to. Everyone else is a dependent variable. (Course they want fun too, but every decision, every whimsy, is product of a character devised to best reflect the protagonist, and that's considering plot and narrative function.) Such great stuff covered already, and loved the triangle idea (the points being protagonist, other characters, and the circumstances they find themselves in) from Kevin Price.