I think it all depends on how you speak up to the GA's or the F/A's about the potential situation before the airplane leaves the gate.

2 years ago I flew LAX-EWR on a sUA domestic 757 and I was seated at a window and the guy who sits next to me was clearly on something. Serious SSRI's mixed with booze/mushrooms/acid or just plain insane - who knows but he was clearly f-ed up. He sits down in the middle, smells like a trash can, and immediately puts his head down on the tray table.

I got up and went to the back and told the F/A that I refuse to sit next to this guy for the next 5 hours and said i was "uncomfortable" with the situation. She sat me in the middle seat of the last row (I think 40?) which was a downgrade but at least I didn't have to sit next to the other guy on a transcon.

I feel with poop it must be worse. I would be livid and threaten to get off and take the next flight or fly another airline.