[MENTION=6919838]5ekyu[/MENTION] while I agree with what you're saying, what if your campaign doesn't revolve around fighting the BBEG (or a big bad evil organization)?
What if it's more of an exploration/old school set of dungeon crawls? I usually end up having story arcs that pit you against different foes or challenges but sometimes I want to run a campaign where it's not about saving the world or even the town the PCs call their home base. I guess you can always add in the "oops, we woke something up" angle, but that only works now and then.
How do you add variety? So that sometimes they're fearing for their own lives and other times fearing for the lives of others?
Well the question posted resolved around keeping the sense of risk alive.
if the campaign is focused on explore etc not fight - then obviously things would be different - xp/advancement not keyed to combat for one.
As a GM if there is no "need" or "stake" in the outcome, why would PCs or anyone stay in a fight where there is risk of death? Are they just suicidal for fun?
Obviously, if they are given no choice, it doesn't matter. if none of their choices can escape or avoid the fight, they have zero control, then hey you have your death match.
But i wasn't assuming that to be the case.
So then you get to "what does matter to the PCs?" That gives you stakes.
if your PCs are "dont give a crap about civilians" then why would they fight in a fight they may lose and die?
Well, if the PCs are focused on exploring and getting into ancient places or secrets, maybe there is a rival team after the same "finds" and then the "conflicts" may not be fights at all but getting around things, finding things before the others do. or maybe the sides are willing to fight - but maybe that fight is a skirmish to kill the horses or distraction while the climbing gear is stolen or a key map stone is spirited away or destroyed.
There is no generic answer to your questions - beyond "what are the characters after? Why are they here? What will they fight for?
those answers should guide a GMs choices of adversaries and stakes, right? It has to be something that matters to the characters.
For me tho, if we were just wandering around for no particular reason in character, any fight we could avoid that wasn't a squash would be one we avoid unless we see a clear reason in character to do so.
Then again, in such a focus on explore game, leveling likely wouldn't be keyed to fighting. Fighting may even be counter to advancing if the time it takes to resolve loses you more "advancement" from exploration time.