The Ultimate Screenplay Format Reference

Unseen Characters

If you follow the rule to write only what can be seen on the screen then why bother with unseen characters?

You do it when these unseen characters affect what’s seen on the screen. Here an example from The Screenwriter’s Bible:
 

EXT. PUBLIC BUILDING - DAY

James Connors hurries up the cement stairs.

An unseen person clicks the shutter of a 35mm camera.
Another click. And again as James rushes into the building.
 

It could easily be argued that only the click is relevant here, even if the unseen person is introduced later. Thus you might settle for the following:
 

EXT. PUBLIC BUILDING - DAY

James Connors hurries up the cement stairs.

A click is heard - like from the shutter of a camera.
Another click. And again as James rushes into the building.
 

The next section provides an example where mentioning an unseen character definitely makes sense:
 

Phantom POV

Sometimes the movie shows the scene from the point of view of an unseen character. That could include a change of color, use of a filter etc. like in Predator. Or it could simply mean that the camera takes the perspective of a character that doesn’t appear in the scene at this time.

The Screenwriter’s Bible makes an example using a monster in the lake. First and attempt at writing the scene without the reference to the unseen monster:
 

EXT. LAKE SHORE - DAY

The children form a circle by the lake.

EXT. UNDERWATER - CONTINUOUS

While the others dance, Pam peers down into the lake.
 

Here the approach including the unseen character - the monster:
 

EXT. LAKE SHORE - DAY

The children form a circle by the lake.

EXT. UNDERWATER - CONTINUOUS

An unseen lake monster watches the dancing children. Pam
peers down into the lake.
 

To understand what’s happening here note the scene heading of the second scene: EXT. UNDERWATER.

That means the camera looks at the beach from below the water. In both cases we see the same events, but the second version prepares the reader for the monster which undoubtedly will appear rather sooner than later.