Joe is an average bloke, who has a passive superpower that he's lived with all his life.
People believe everything he says. Implicitly, unquestioningly and absolutely. If he says that black is actually neon purple, people within earshot accept that as a natural truth. If he then recanted his statement then black would be black again.
This has certain advantages ('But this meal is on the house, waiter!'), but Joe wants to live a normal life (for a variety of reasons, one of which is evil assassins).
How can Joe avoid accidentally using his superpower in everyday life given that even seemingly innocuous statements like 'Good Morning' can completely change the mental state of those around him?
Joe can't turn this power off, and wants to be able to live as a normal western adult male, so taking a vow of silence or retiring to the Rockies in a tent aren't options.
Edit: For the avoidance of doubt: This applies only to spoken and heard communications (the deaf are immune). On the other hand: For the purpose of combinations such as pointing and speaking, anything that someone would think to themselves 'that's obviously a lie' when it's said/done by non-Joe, believes it implicitly when it's said/done by Joe. If the person has misheard or misconstrued what Joe said, then they implicitly believe what they heard/understood, not what Joe intended. If what Joe says just doesn't make sense (for example: Patent explosive donkey muffin discombobulate.) then confusion is the response that will occur, not attempting to parse the sentence into 'truth'.
Now I want a Patent explosive donkey.