So, here's the situation. You're playing in a friendly game against someone who is either a) trying an army for the first time or two or b) an inexperienced player in general. In the course of play, they make major errors that benefit you. You may notice that they're about to start moving guys and therefore forfeit their opportunity to roll reserves for that DP dread sitting off-table or about to charge your T6 Chaplain with guardsmen...
How do you handle this situation? Do you remind them politely that Cassius is T6 and they'll not easily wound him, or do you stop them from touching that Rhino and say "reserves?"
Now that you've answered, apply the situation to a tournament, where most mistakes happen because we're hurrying so that we can get in more game during the time limit...do you stop your opponent from doing something silly, or say "Dude, you don't want to do that".
Did your answer change?