This is one my absolute favorite exercises. Here’s how it works:
Go someplace where there are people, (a diner, park, a mall, a coffee shop or the street). Look around and find someone who takes your attention, who interests you for some reason.
Now write about that person...without speaking to them.
Just start noting down whatever comes to your mind as you look at them. Tell a story about the person. It doesn't have to be true. You can do some physical description, but in the end you’re concerned with what you imagine from the details you see.
Once you’ve worked with it a bit, look over what you’ve done and see if you can use it to write the opening of a short story.
People are surprised by this exercise, and a little nervous. After all, aren’t we here to do photography? So I tell them that it is an exercise in seeing, not craft, and that they shouldn’t try to write it well. They can even make lists if they want to. But the funny thing is that the pieces are always well-written, by which I mean simple and direct and effortless.
The point gets made that these vignettes—call them street portraits—are like cameraless photos, and just like actual photos, they begin with seeing, not with picture-taking. When a class reads them and sees/hears what they’ve done, it makes that point sparklingly clear.
Read a few examples here, then try some on your own.