There was a beeping noise coming from one of the storage units.

The woman investigating it appeared to be the building's concierge or maintenance person. She found the key, entered the unit and discovered a man beneath a sheet, strapped to a gurney, handcuffed, bandaged and connected to medical equipment.

Then he woke up.

I had walked into the living room halfway through the film. My boyfriend had been watching it for some time. I hadn't seen the beginning. I didn't know the characters, plot or even what film it was. Within perhaps a minute, I wanted to know what happened next.

That accidental experiment produced what I now call the Walk-In Test.

Imagine somebody walks into the room halfway through your scene. They know nothing about your screenplay and have absolutely no obligation to watch it. Does something make them stop?

The storage-unit scene captured me because every discovery altered what I thought was happening. A beeping noise leads to a locked unit. The unit contains medical equipment. The equipment is attached to a man. The man wakes up. Instead of providing answers, he knows even less than the woman who found him and mistakenly believes she is responsible.

Curiosity isn't created simply by withholding information. Give the audience an answer that creates a better question.

The scene also demonstrated how little an audience initially needs to know about a character. The woman had a problem and dealt with it. Her behaviour told me enough. Give five characters the same locked door and one breaks it down, another searches for the key, another listens through it, another calls the police and another walks away. Character is revealed by what somebody does when something gets in their way, their biography can wait.

Audiences are remarkably capable of catching up, and working things out is part of the pleasure. During a rewrite, try removing the beginning of a scene. Lose the arrival, greeting and explanation. Start at the first decision, obstacle or anomaly. If everything still makes sense, you haven't damaged the scene. You may simply have found where it should have started.

Then use the Walk-In Test elsewhere, two people drinking tea can hold us if we're wondering whether she will tell him, whether he knows she is lying, or why neither of them will mention the empty chair. The question can be tiny.

I walked into a film halfway through with no context and no reason to care, within a minute, I sat down.