Living where I live it used to be guaranteed battle stations any time the Old Firm met. It's not as bad these days, as the Scottish contingent here gets more watered down by the influx of Poles/Serbs/Lithuanians/Hungarians etc. In the 60s and 70s if you wore the wrong shirt in the wrong pub (or wrong part of town) you got the shite battered out of you, no hesitation. And if you were especially arrogant or unpopular, a blade would be used as well.
Aggression and brawling were such a way of life that it took me quite a while as an adult to realize that it isn't the automatic answer to any situation that arises, just to belt some cunt's teeth out.
As much as I disagree with the sectarian crap that goes on, there was a kind of code of conduct that the two factions saved it for each other come derby time, and didn't just run around lamping any innocent looking gadge.
You see them now as they are older, and you wouldn't know it. But come derby day, and the beer starts flowing (usually around 9-10 am) the old hatred comes to the surface again, although they're generally too old and fat to be getting physical with it.