What do you want your readers or audience members to feel?
Too often we forget that the best reading or movie-going experience is an emotional one. We are paying to be moved to laughter, to tears, to fear, to wonderment.
I'm talking mainly about fiction writing, but it applies to some degree to non-fiction, too. Yes, we want to be informed but that is much more likely to happen if our emotions are engaged as well.
Advertisers know this, of course. They want you to take action--to buy something, usually--and they know that you'll do that only if you're motivated. The strongest motivation comes from feeling a strong emotion. It may be that if you don't buy product X you won't be popular or attractive, or if you use product Y your days will be joyful.
Unfortunately most of the products don't really deliver, but it's important that our books and screenplays do.
Therefore I suggest considering this question on several levels:
What do you want your readers/audience to feel when they have finished your book or seen your film?
If they leave with a strong feeling at the end they are much more likely to continue to think about and talk about your work. The strongest word of mouth comes about when people want their friends to have the same emotional experience they did.
What do you want them to feel about each of your characters?
We may want readers' feelings about a character to change, in which case we have to be sure to give them sufficient reasons; as in life, in fiction a first impression is powerful.
What do you want them to feel in each of your major scenes?
Here we have to take into account not only the emotional tone of each scene in isolation but also in light of what went before. An emotion is stronger when it is felt after its opposite. That's why good thrillers and horror stories often juxtapose a funny or calm moment with a shocking one.
If you find it too inhibiting to think about this while writing the first draft, use it as a rewriting tool. I hope it helps you write the best book or screenplay you can--that'll lead to some great positive emotions!