The green man dies. Write that scene. How does this effect the main character? Is the main character present when he dies or find out later? If they find out later, write that scene.
How does the green man relate to the main character? Write a basic plot line of their connection. Then write a scene depicting how they meet.
Since the green man isn’t the main character, you’ll need to invent one. Write a character sketch to tell us about them.
The green man isn’t the main character, he’s part of the supporting cast. But he’s got an important role. Explain why he matters to the world he lives in. While you’re at it, paint a picture of that world.
You now have most of the work done! Write the tree guy’s story.
Now, where does the story begin? What is the major conflict? How does it end? Write a scene that highlights each of these important pieces of the story.
Now that we’ve written a bit about the tree guy, write a scene where he encounters his enemy.
Did you do the prompt on the tree guy? Now write a scene that gives us a sense of his character.