You can turn a contest into a battle by invoking Hubris. What if the contest is something like, crossing a river. Does the GM then have to create the river (or something proving adversity) as an NPC, to play out the battle?
Or does this rule only apply when you're rolling contests against living or monstrous opponents.

Personally, I would totally make up the river as an NPC. It's in keeping with the myths and it would be fun as hell.

"The river attacks you with its 'Drag You Under' weapon!"

Awesome.

Yes, I think so too :)

A related question prompted by the example in the book of the heroes who are trying to sneak into the temple.
One of the Heroes succeeds, the other two fail.
Now, if one of those two doesn't want the embarrassment of failing and invoke's Hubris,
a) does the hero who won have to be involved - he achieved his goal, so are the others rolling to simply join him? If he does get involved and gets defeated, he naturally loses the goal (or does he?). So he's probably very keen to know if he has to take part.
b) Does the other hero also have to take part? Could he accept failure and ignore the Battle?

The overriding question here is: if one hero calls Hubrus, does he automatically involve the other heroes present? That opens up some interesting tactical decisions, if so...