"The movie was very scary."
Japanese -i adjectives show tense, so you would never use "deshita" with an -i adjective. -na adjectives do not show tense because they behave like nouns, so you would use "deshita" with those.
Past Tense of -i Adjectives
I was busy.
Past Tense of -na Adjectives
I was free.
Why the "that" is in the answer with "the" listed perplexes me. No context from previous conversation to change it to sono
(I don't know if it was intended, but you left out とても in your translation.) As for こわかったでした, that is not right. It is like double past tense in English ( I did ate). So it is either その映画はとてもこわかった (informal/plain form) or その映画はとてもこわかったです (formal/long form).
I wish they'd just use こわくありました instead.
All this かったです, ないです, and なかったです just looks wrong when reading it. Even こわいです is bad enough. I know these are normal Japanese though... xD
Japanese doesn’t have a definite article, and その can serve a similar function to “the” in that it points to a specific thing. That said, if you write this sentence without その it should also be accepted, and if you’re doing the English translation, you should be able to use “that”.