I've seen both the good and the bad.
- everyone in the room is dead, guys realize that they wouldn't have survived a grenade, call the "hits" whether they were actually hit or not.
- guys duck behind a plastic chair or miraculously not get hit by an actual BB and thus are completely unscathed...
Just make sure the host for the night (or whomever is running the game) makes a clear rule about them before the game starts. Without it stated at the game brief or in the rules for the night, how you might expect them to work vs. anyone else here vs. some random walk on to the field vs. etc....may be very different.
It's also a time to allow people to voice any questions/concerns before they are used.