The protagonist’s journey is the A-story. The audience will only care about the story if they sympathize with the protagonist. That is, if they care about what happens to the protagonist, they will care about the protagonist’s story, and thus the script/movie you’ve written. If the audience doesn’t care, then they disengage, and then they either play on their phones or go watch/do something else. To engage the audience, the story needs to elicit their sympathy for the protagonist. There are several common ways to do that.
Undeserved misfortune. The audience feels bad for characters when something bad happens to them. The key in this case is if the misfortune is “undeserved,” i.e. the character doesn’t do anything to deserve the bad thing that happens to them. Otherwise, it comes across as karmic justice.
Undeserved misfortune is the reason why so many protagonists in so many screenplays have recently lost a spouse and/or child. Killing loved ones is a quick, simple, powerful way to elicit a sense of undeserved misfortune. We might also say it’s a little cheap, given just how often we see this powerful card played; screenwriters have killed more spouses on the page than the Bubonic Plague. But tropes are tropes because they work.
Death isn’t the only undeserved misfortune. Very often, the misfortune comes from the actions of the antagonist, for example the very Joseph Campbell-ish choice of having a monster/warlord attack the protagonist’s village. Or the protagonist is unjustly accused of a crime, left at the altar, robbed of something they worked to create/deserve, and so on.
Simply put, dreaming up ways to torment our imaginary friends on the page is one strong way to establish audience sympathy.
Laudable goal. We sympathize with characters who are trying to do something that we find to be laudable. We sympathize with the sheriff who is trying to stop the giant shark from eating people because giant sharks eating people is bad, and stopping that from happening is thus a good and worthy mission. The audience can also get behind a more pro-active goal, whether it’s personal (trying to find True Love) or more broadly social (improving the world in some way).
This is why it’s vital to establish a sense of stakes in the story. The strength of the stakes directly correlates to the protagonist’s goal, and thus how we feel about the goal, and whether or not it elicits sympathy.
Note that sometimes the goal comes from a darker wish-fulfillment place. For example, revenge is a very common motivator in thrillers and action projects. The protagonist suffers a bout of undeserved misfortune from the antagonist, so we’re not only sympathetic to the protagonist, but then doubly sympathetic because we’re 100% behind the protagonist’s goal of getting revenge against the antagonist for the undeserved misfortune.
Likability. If we would like the protagonist in real life, we will like them on the screen. And if we like the character, it’s easy to sympathize with them. The quickest, easiest way to get the audience to like a protagonist is to make them funny. As human beings, we almost automatically like people who make us laugh.
But the ability to Write Funny can be difficult; it’s a much less common skill than many people realize. So another way to establish likability is by establishing for the protagonist a clear, strong moral framework. That is, the protagonist shares our values; the protagonist can be counted on to do the right thing in a given situation. The audience’s ability to connect with the protagonist is often dependent on the protagonist saying/doing the things the audience would say/do in the narrative situations presented on the screen. If the audience is thinking, “What the hell are you doing…?” then we lose them. But if the audience is thinking, “You tell him!” then they’re on the protagonist’s side, because the protagonist is expressing their shared moral beliefs.
Note that this can be aspirational. That is, a character does the right thing in such a way as to tell the audience, “This is the right thing to do.” We’ll often find this kind of behavior with a character we might call a Pure Hero, e.g. Captain America. While with other protagonists, the shared moral values are expressed by the thought of, “That’s what I would do in that situation,” with the Pure Hero the shared moral values are expressed by the though of, “I hope I’m good and brave enough to do what the hero is doing in this situation.”
Transitive property. This source of sympathy is often used for a protagonist who is by themselves somewhat unlikable in terms of personality. That is, the protagonist might be driven by a laudable goal, and share moral values, but their personality is abrasive to the degree that the audience might still struggle to connect with the character to the degree that elicits sympathetic engagement.
So what the script/story does is create a character the audience does like. For example, the protagonist might have a supporting protagonist who is funny, and thus likable. Then the story shows us that the likable supporting character also likes the protagonist. Thus, likability becomes a transitive property. We look at the relationship and think, “Well, I don’t like the character, but this other person I do like… likes the character. So apparently there’s something to like.” This approach doesn’t guarantee audience sympathy, but it’s a way to get the audience to at least give the unlikable protagonist a chance.