How would you react to an E2 who "smart mouths" you in formation?

Nip it. Establish dominance. All kinds of unpleasantness. Duty on weekends and evenings. Pet project kind of a thing.

Front leaning rest position.... MOVE! >80)

Bottom line up front - I would dismiss my formation WITHOUT tasking anyone else, and tell the troop to stand fast. He/she will have 30 seconds to explain why they told me to shove it, just in case they had a brain to mouth misfire. I do not care if there is any personal issue happening with the troop, one does not address an NCO in that manner, in front of other soldiers, and that at a minimum, a summarized art 15 will be recommended. If the troop is having a serious issue, the Summarized can be suspended, provided another incident like this does not happen again. If the troop is just being defiant, well, that recommendation will upgraded to a company grade at a minimum, and I would seek advice from the PSG, who would probably want to speak with the troop as well. It is one thing to find out what is going on with your Soldier, quite another to allow that Soldier to openly and brazenly disobey an order, with such a blatant disregard for decorum. We all have bad days, I get it, but there is a right way and a wrong way, and this is beyond just being the wrong way.

When I was a young Corporal, I put a Marine on the ground who did that. I got in quite a bit of trouble. My platoon sergeant GySgt Melton called me into his office, ripped me a new one and told me not to do that where others could see it. ;-)

Keep in mind this is 2021 and a different military. That E5 will have to get in touch with his Mom and ask permission to rectify this problem

Back in the day I would probably have looked at him in a cold and calm manner, then said something along these lines: "Seaman Staines, you are obviously operating under the false belief that obeying the lawful order of a Petty Officer in the United States Navy is an optional affair. This is your work assignment and you will complete it to the best of your ability in a timely manner or you will suffer the consequences. Now shut your face before I kick a mud hole in your ass and stomp it dry."

Now that I am a man of God, I would simply advise him to give his soul to Jesus because his butt belongs to me, and then proceed to make his day very unpleasant; all the while praying for his sorry soul.

I would suspect that basic training and AIT aren’t what they used to be for this to happen. But remember that the boss has no friends and too much socializing with subordinates can undermine your authority as well. Some private individual counseling is called for to make sure there are no extenuating circumstances, but then a good ass chewing is probably in order. I really had no problem with the DI’s as my old man was tougher on me than they ever thought of being.

Times have certainly changed since I was in 50 years ago. At that time you would do fifty push ups, taken out behind the barracks and get an ass kicking. After that would be K P and every crap detail for a month. Sure seemed to work.

I have been in this situation as an E-5 to a Buck Pvt. He told He didn't want to do something. I stopped and turned around and told Him I don't remember asking you. Now if you don't want to clean all the latrines in the barracks get the task done which He did.

tell them to fall out to first shirts office and then continue allotting work