When the proctor says "pencils down" does that mean you have to put down your pencil right away, or can you just quickly fill in the few empty bubbles on your sheet? I know you're probably "technically" not supposed to do this, but will I get in trouble?
No, it is not okay. The very first item on the list of ACT "Prohibited Behavior[s] at the Test Center" is as follows:
The following behaviors are prohibited. You will be dismissed and your answer document will not be scored if you are found:
"The following behaviors are prohibited. You will be dismissed and your answer document will not be scored if you are found:
Filling in or altering responses on a test section on your answer sheet or continuing to complete the essay after time has been called on that test section. This means that you cannot make any changes to a test section outside of the designated time for that section, even to fix a stray mark or accidental keystroke."
So yes, filling in answers after time is called is considered cheating. But will you "get in trouble" for doing this?
Unfortunately, the answer to this depends on how well-trained your proctors are. I've seen students claim they've gotten away with bubbling in answers after time was called and students whose scores were voided because a proctor (or another student) reported them. I say "unfortunately" because this means that not everyone has an equally fair ACT testing experience. Ultimately, I don't think the risk of being caught and having your entire test invalidated is worth the reward of getting a possible extra point for bubbling in an answer after time was called.