As Shakespeare said, “strong reasons make strong actions.” The reason this is so profound is because if you don’t know why a character, especially your protagonist, is doing something or wants something, you will have a rambling screenplay.
Your plot will be unclear and your dialogue, no matter how clever, will mean little once your audience starts wondering why your main character is saying these things.
An old rule of writing: every line of dialogue should reveal character, move the story forward or get a laugh. If it doesn’t do any of those things you need to rethink it.
And it’s not just one thing in the script. You need to justify every line in every scene. If your protagonist goes into a dry cleaning store she better have a reason. If it’s just for her to pick up eleven dresses so we can see that she has excellent taste in clothes, that’s not enough of a reason for the scene (unless her taste in fashion is key to the character and plot). We can learn that by looking into her closet.
If she has an exchange of dialogue with the proprietor that’s about the difficult stains he was able to remove—-not enough—unless it’s a thriller and as he’s telling her how he got the stains out it triggers a memory that will move the plot forward.
If you have her go into that dry cleaners it must be for a reason germane to the plot or her persona.
I have a friend whose mother is super grandma. He has four kids under twelve. When he was growing up he didn’t have a grandmother because his mother’s mom died when she was a child. So his mom’s children (my friend and his two sisters) grew up without a grandmother.
When my friend started to have children, along with his two sisters, his mother very quickly became the greatest grandmother in the world.
I’ve known this woman all my life. I asked her about being super grandma and she said that her kids had to grow up without a grandmother and she didn’t want that to happen to her grandchildren.
How’s that for a strong reason resulting in a strong action?
Whenever I have trouble pinpointing why a character is doing something, I think of her. And it helps me concentrate and giving a character proper motivation.