In the first, double-length episode "Serenity" of Firefly (not to be confused with the film Serenity), Mal and his crew attempt to cut a deal with the dodgy Patience. Before their meeting, Mal sends Jayne around the valley to deal with the snipers that he's sure Patience has in place. We see a few cuts between Mal talking with Patience and Jayne watching through the sights of the sniper rifle. When it becomes clear that Patience is out to get them, Jayne starts the shooting.
How did Jayne know exactly when (or whether) to act? Sure, he could see through the rifle sights that Mal had just tossed the money back to Patience, but there could be other reasons for that - it was too little, or he was just checking it before their exchange of goods, or they'd amicably decided to back out of the deal. If that had been what had happened, it would've been Mal's side who started the violence unprovoked.
Out of universe, of course it's "cool" to have the hero complimenting the enemy sharpshooter's hat and the henchman shooting it off right on cue. I'm looking for an in-universe explanation.