Sometimes, drivers stop for a short time at a snack bar, but not always. Have your passport easily accessible. Police board buses as often as every couple of hours in some parts. This is Mexico's fight against drugs, I'm told. The uniformed officer may request identification.
Passports are preferable.
In his book, Carl Franz says officials also spot check driver capability and credentials. Porters meet every bus, in my experience. Without them, I couldn't have maneuvered around the large stations. I tip according to service rendered, time of day, etc. As he guided me to the appropriate counter, he told me there are seven terminals within this bus station.