I've fished big rivers in PA for flatheads and lakes a few times in Ohio. Our favorite spot was a big, deep eddy on the upper Allegheny River. 99% night fishing from a boat, and we mostly use big suckers (up to 12") for bait. I know a few creeks where I can seine them. We have used soft-shell crayfish (we call them crabs), with decent luck but suckers are better. I know some hard core flathead guys that swear bluegills are the best bait but I've never tried it.
We've caught some nice ones but never got any big ones. We've had some 20 fish nights. Our biggest was 36", 25#...nice fish but nowhere near what a top-end flathead can be. PA lagged behind some other states for the record size. For a long time, it was 44# but the record was broken. Not sure what the current PA record is.
Ohio has some big ones. For years, the record was 73# caught in Clendenning Lake. 50 pounders are not uncommon in the Muskingum lakes. The boathouse in Piedmont lake has one mounted on the wall...55#. It's really impressive but some states have records around the 100 pound mark (maybe Texas??) I can't imagine what a 100# cat looks like in the water.
I love to fish for them but I am by no means an expert. Pretty much like whitetail hunting, I have my "comfort zone" on how I do it but the really successful guys are in a different league.
So, from what I read, flatheads spend the daytime in deep holes associated with structure. The do like hard bottoms. At dusk they start moving and feeding. 1st in the deeper water and then eventually move into to the shallows later at night. Maybe that why we've never done great on them late in the night...we always tended to continue fishing the deep holes. Maybe the cats moved into shallow after midnight and we were still fishing deep.
Flathead fishing is a rush. The minute or so while one is running before setting the hook (or not setting the J hook) is so exciting. The thoughts that it could be a 50-75 pound fish is so exciting.
Side story...
My wife had a run one evening and she set the hook and had the fish one momentarily, then it got off. She reeled in to re-bait but when it got back to the boat, not only was the 6" sucker still on the hook, there was now a 2nd, half-digested sucker on the hook. We surmised that the cat swallowed her bait but when she set the hook she set it into the other sucker that the cat was digesting and she actually pulled it out of the cat's belly.