The movie was really scary/scaring я знаю, что ответ - scary, мне он неважен мне необходимо адекватное объяснение, почему здесь scary, а не scaring не игнорьте, (

SCARING:

progressive tense requires form of be:

is scaring +direct object (am/are/was/were)

The moving curtains are scaring me.

Gerund is using the word as an activity (noun)

Scaring children is not nice. (the action of scaring children)

SCARY is an adjective:

Scary children are not nice. (children who scare)

This movie is scary.