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There are four roles, which are "rules important", yes. But I just realized "Party Leader" as such isn't one of them! I was thinking of the Guide.
The four roles are:
Guide: Must be exactly one. In charge of planning the journey: keeping the party on course, deciding when to stop for a rest, how to manage supplies, etc. So it's easy to see this as a "leader" role since they're making the decisions, but it could also be the person the leader delegates travel specific decisions since they're the expert, while the leader makes higher level decisions. (Or the group could all make a show of deferring to the jackass who appointed himself "leader" while secretly laughing at him behind his back and listening to the Guide instead. Or the group could be organized as a cooperative with no true leader, who agrees on a guide for each journey out of convenience. Or the guide could be a character whose personality doesn't lead them to tell people what to do, and the party can theme the Guide roll as other people constantly asking their opinion since they have the best wilderness skills. Etc.)
The Guide's main asset is a high Travel skill.
Scouts: Can be any number of these. In normal travel, they check for obstacles ahead, or search for things that aren't right on the trail (good campsite locations and lookout points, etc.) If you're forced to leave the trail for some reason you may need a scout, otherwise they're optional. If you don't have any scouts, you don't have much flexibility or options: you deal with what you encounter on the road since nobody has searched for ways around.
The Scout's main asset is a high Explore skill.
Hunters: Can be any number of these. They find food. If you don't have any, and you run out of provisions to carry, you start starving, but if you're carrying enough provisions for your journey they're optional.
The Hunter's main asset is a high Hunting skill.
Lookots: Any number of these. These stay with the main group, keeping their eyes peeled for ambushes. If you don't have any you're vulnerable to being surprised.
The Lookout's main asset is a high Awareness skill, but you can never have too many lookouts, so anybody not doing one of the other roles should be on lookout duty.
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