As I have mentioned in the past, I had a player, in different games, behind the shooter and in the space next to the shooter squeak their shoes. Here is the kicker. One time the shooter stopped and looked at the squeaker. (disconcerting?). I held out my hand and called a violation when the free throw was missed. I did not have the time to quietly warn the squeaker.
What about the Coach or Bench personnel yelling during the free throw? I had a Coach yell "Get the Rebound" and it was the 1st of 2 throws.
I try to inform the players. " Hands up or down. No movement or talking when the player has the ball."