Learn English – Do I need a comma before “since” in this sentence

I don't think we need a comma before since. When since is used as a conjunction to mean "because" at the beginning of a sentence, we use a comma at the end of this dependent clause. However, it's not necessary to put a comma before since when this dependent clause starts after the main clause.

We are not referring to a particular emotion. Instead, we are referring to all type of emotions. So we must use "emotions".

I don't think there is anything unnatutal or non idiomatic in the sentence in question.

Yes and no.

If you were going to include 'in this case' you would write:

This was the first time she played the main character in a film, the movie of her life, in this case.

But to the point, 'in this case' doesn't sound correct here and isn't needed at all. So:

This was the first time she played the main character in a film, the movie of her life.