As an In-Room-Dining server, we earn less than the national minimum hourly wage. So we depend heavily on the generosity of guests with regards to additional gratuities. I say additional because there is a 20% gratuity included in each check. That gratuity is actually divided among the staff working that day (servers, captain, order takers, etc.).
The additional gratuity written on the check by the guest actually goes directly to the server who delivered the order. This usually leads to some healthy competition among the servers. They each have their own way to encourage guests to give them an additional tip (whether it's written on the check and given directly to the server in cash form).
I work with a guy who is obsessed with money. This unhealthy obsession has led him to stealing orders from his co-workers, bad mouthing his co-workers to the regular guests and many many other despicable acts. He is so busy trying to sabotage his co-workers that his own performance level has declined.
Now, he has resorted to actually writting in his own gratuity on any check where the guest did not total up. If the guest simply signs the check but does not total it, he just writes in an additional tip for himself.