The guy you filled in for is a 5/8, I'll assume. If they paid him OT after 40 hours, he would still get 16 hours OT (4x12=48+8=56 hours). If they started making stations cut back on OT, he would be going home after an 8 hour shift. If he's 4/10, he gets 8 now and will get 8 if it changes.
We have people like that in my station. PT'ers working 50-60 hours, FT'ers working 10-12 hours on Saturday, etc etc. I keep telling them, don't get used to the hours, they won't be there forever. For me?....pay me my guaranteed 35 and I'll gladly go home at 2:00.