This came up on Facebook in the "euphonium players" group. Here was my answer; please add anything I might have missed or correct me if you disagree with any point(s):
The principal player has responsibility to A) know the parts very well, B) to make sure to hear the rest of the ensemble and understand how the euphonium part fits, C) to guide the section by example, D) to guide the section by discussion where appropriate, E) to choose who plays in solo or a2 sections, F) to make sure the players in the section pay attention and remain respectful when the conductor is talking or working with other sections, and G) to work outside of rehearsal with the section where needed to make sure all players blend and fit together. That's most of it, anyway.
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