From the FO side, I recently ran into a day where the delays took us right to the edge of the duty day. The CA was called a few hours before the end of our duty day and advised of the duty "plus 30 minutes" time and the absolute "drop dead after 2 hour extension time". I, the FO, was never called. At 10 minutes prior to timing out, I called crew scheduling and asked what the plan was. They gave me the same times and said they were too busy to call everyone. They would just "assume" that I would work the extension unless I called them first. After a little research, they are only required to call the PIC (Captain) to ask for extra innings. (On this particular day, the CA said he was never really "asked" if he would go into overtime, just told the times.) Unless the FO claims fatigue, you are along for the ride. As previously stated, a fatigue call will put you in touch with a member of management but that is nothing to be afraid of. A good CA will ask the crew if they are willing to go into overtime before telling scheduling yes.

They are careful not to "pilot push" because they have lost legal arguments for it (I met one who claims to have a gotten a sizable little settlement for said act), but sometimes they can get closer than is comfortable. The "assumption" that you will go into overtime is what I don't like. If they called and asked "Say ... it's been a rough weather/maintenance day ... can you hang on for one more leg? Would you be willing to get this plane to the overnight?" I'm the kind of person that will help when asked ... the "assumption" is what ticks me off.