NBC's The Good Place asks the hard questions about death, immortality, and motivation.
There are a few things that are guaranteed for us, and one of those certain things is death. All the mortal characters in your story are going to die eventually. Maybe not within the brackets of the plot, but beyond the page, their days are numbered. When we talk about want and motivation in narrative, we don't always discuss that death is always upon the horizon. But in some stories, it's the center focus.
We aren't entirely sure what happens to us after we die. That's not a knock on your creed: whether you believe in heaven or nirvana or anything else or nothing, we can't really be 100% sure. There are no phoenix downs that have brought real life people back from the dead to tell us the definitive answers. It's the unknown side to our existence. The end. If one thing is for sure, we won't have direct affect on the physical world when our time has come to an end. This is what drives urgency to our characters.
Imagine, for a second, if your main protagonist found out they were immortal. What if, as they were mulling over the resolution in the second act, they realized by some twist of action that they weren't going to die. Then what? If your main character is immortal, would they continue their pursuit? Because without death, then we don't really have to try.
Everything we do is predicated on our need to survive. Deep in its core, the very reason I'm writing this blog is to get hits to the website, which may turn into potential clients, which means I can buy food and pay rent. It's not the only reason why I'm writing this, though, and I think it's important to create characters who aren't caught in conflict merely for their own survival.
Selfishness and selflessness ride on the same spectrum. No one is entirely selfless. No heroine has ever gone out to save the world for the sake of the world alone. She's invested in saving the world because there's something personal in it for her. If we can't identify what that personal investment is in our conflict, then we're left with a simple, cliche plot: save the world or be killed.
In NBC's The Good Place, the characters Eleanor, Chidi, Tahani and Jason are threatened to spend their eternal afterlives in "the bad place." But as it stands, they are in the good place, everything is fine. They wouldn't have to do anything in this utopia. But when the dysfunction of the utopia becomes more and more apparent, putting the characters at odds with each other, they soon realize that they are actually in "the bad place" and have to do something to get out. But why help each other? Why make any sacrifice? The bad place isn't the place they thought it was, and it's not a threat to their lives because, well, they're already dead.
Sure, there's psychological and physical pain at stake. But there isn't any loss. With immortality, there's no threat of loss of life. Death (along with pain as an end result) can't be the only threat to your main character if you want a full, compelling narrative. What is she going to have to live with, and eventually die with, if she fails? The loss of a friend or family member? The loss of her free will?
If it's loss of a person, that person doesn't necessarily have to die, either. That person could be taken away from her, possibly forever, while still being alive. I know I reference The Hunger Games quite a bit, but if Katniss is immortal, then she enters the games and has nothing to worry about. It's not death that scares her, it's her sister getting hurt or worse in the games (something she definitely doesn't want), which means Katniss would have to live with that loss (another thing she doesn't want), thus prompting Katniss' decision to volunteer.
The Good Place does find effective ways to finagle loss into its plot. And that's the important part here: if you're character fails, what does she lose besides her life? Where does she reside on the spectrum? Does loss experienced by another character resonate with her? Why? Does it affect her? How? And what is she going to do about it?
Death is what makes us do what we do, but it isn't the only reason why we do the things we do. We're invested, and we want things to happen (or not) because we care about not only our own well-being, but for the well-being of others--depending on where we stand on the spectrum. Identify what your heroine is invested in, inside and outside of herself, and you're on your way to writing a vitally divergent character.