Not only that, I often witnessed people managing to keep a low profile on services between Sheffield and Leeds to avoid payment - on at Swinton, off at Outwood for example, I saw another the other day, already on the EMT train when I got on at Manchester Oxford Road, paid for a Piccadilly to Sheffield ticket when challenged - I've no idea how train staff remember who wasn't on the train before their last check, often they appear to rely on passengers positively presenting tickets on the busy services - plus at peak periods, there's little hope of being able to walk along the train. For me, barriers are the way of making sure my fellow scumbag free riders don't escape payments.
In a nutshell, you basically have it. I certainly can't remember on a full train with 300 + passengers who was and wasn't there. I occasionally ask on instinct and get lucky, or have to make an embarrassing 'ah yes, I've already seen you' climb down.
Generally just presence helps and I occasionally operate entrapment - I always go down between Lime Street and Manchester as I'm certainly not scared of the Widnes scroats and there's lots of money to be made out of chancers. However I sometimes leave one of the first two coaches unchecked on purpose and then do a full grip after Man Picc and it's amazing how often you get a 'you've already seen it guv' bite on the fishing line.
I also always do a full grip after a crew change, ignoring the moans of Doris and Ada at having to get their advances out again for the second and last time in 5.5 hours.
I'm actually quite pleasant and calm in real life but I do enjoy making the odd fare evader's life a misery
Just for confirmation - I do not tend to grip Stockport to Man Picc as I've enough to do checking connections and being at the rear for those horrible narrow platforms (if someone does go under I want the best chance of receiving a stop signal). I do it from time to time though to keep folk on their toes though !