Hello,
I just signed up on these forums to ask a quick question.
I've been lurking here for a while now, and this site seems like a top hit for google search results

First of all, are these terms used at all in formal writings or do you only see them in informal texts such as dialogue?
"Heck, I don't even know."
"Hell, I don't even know"
"I mean, it could be possible."
Is this even correct? I know "heck"/"hell" can be used as in "what the heck" and "what the hell"/"what in hell", but can it be used as an intensifier at the start of a sentence and immediately before a comma?
That also brings me to this – is there an official term for these words or phrases? If not, is there a type of term to use as a substitute? I'm having trouble thinking what I could use in place of this, but I really want to add some depth and thought into a certain piece of writing. I'm trying to capture the feeling of something crazy or unbelievable.
It seems to me like these phrases are informal, because they show the 'thoughts' of the writer.
Sorry for asking so many questions, but it's hard trying to describe something unfamiliar without knowing its name.

Thanks.