premise indicator
If you encounter someone that is sleepwalking are they acting out a dream?
Generally, the answer would be the subject, a sentence must have a subject and a verb. However, the exception is an imperative sentence, when the subject 'you' is implied, not expressed; for example, "Help!", "Watch out!", or "Look!" A noun clause may consist of a subject noun or pronoun and a verb but as an incomplete idea, it is not a sentence.
she was acting primly :)
asterix was an animating animal that appreciated artful acting
She has been acting funny lately.
That kook is acting out again.
Yes, for example, 'acting' is a verbal noun (gerund). Example sentence: I plan to study acting in college.
The people were acting as if they were Brutes
she was acting out as i was a hard tack
Example sentence - He hasn't been acting like himself lately.
The class was acting up again, and the teacher knew she would be giving out detentions soon.
No, it is acting as a pronoun (that thing, whatever the thing is).