The hostility depends on many factors. Some areas are more hostile than others, and individual spots. Here are some things that could lead to a negative atmosphere developing, from the point of view of a non-local surfer arriving in the lineup:
- snaking (cutting in line)
- dropping in (taking off when someone is on the wave)
- sitting at the main take off with the locals (many spots it takes time to earn a place there - years where I live)
- arriving with more than 2 other people
- hassling people, putting on pressure, being too hyped up (chill and wait your turn)
- having photographers if it's a sensitive location
- just paddling out at all if it's a sensitive location
- getting in someone's way (when paddling back out and someone is riding an oncoming wave, paddle into the whitewash rather than paddling towards the shoulder)
- being loud (for many folks surfing is a bit like meditation)
- wasting waves
- being out in waves that are clearly beyond your level of experience
You don't even have to do this yourself. Someone else can and then you cop the flack. It really depends on who is local to the spot and what they're like. If the vibe gets negative then just head in and paddle somewhere else. If there are particularly grumpy locals, other locals will probably understand your position if you're cool about it.