The easy ride to Bishop was a lie

Sam gets duped into buying a mule, wagon and supplies instead of taking the stagecoach. The route ahead just became much more difficult and Em rebels. This is the first indication that she's willing to abandon her family for the comfort of a nice hotel.

This is the first hard turn in the story. The family is no longer traveling; they are improvising. Sam tries to act confident, but the audience should feel how thin that confidence is.