Please be aware that one of the symptoms is hyper-vigilance. That means they have a high startle response and are hyper-aware, whether consciously or not, of changes in their environment.
It is NOT OKAY to repeatedly poke someone so you can watch them scream and flinch.
Or walk up behind them and scare them.
If someone you know has PTSD, despite how “funny” it may seem in the moment, it is not nice to activate their startle response on purpose. It does not feel good for us and can sometimes take a while to calm down.