The off-stage scream terrorises.
We are caught off-guard. On-stage actions cease.
Whose scream is it? Why has it occurred? What will the consequences be? What awful truth awaits us?
What does it mean?
It means things occur elsewhere.
It means we have been diverted.
It means we have been looking in the wrong place.
It means we are unready.
It means there are things we do not understand.
The writer or director has held back information. Or lied.
For a few moments we are convulsed with the realisation and horror of our own not-knowing.
The beheading was not filmed.