Getting ready to deviate on the front of a rotation.

What (if any) obligation do I have re backup flights (a la "unable to commute" and/or "call in honest" for commuting)? Does that answer change if one (or more) of the planned flight(s) is offline? DCI?

I guess I'm asking, does having a PS seat change the equation? "What if" the flight I'm PS on delays or cancels--what's my status?

[I get that the scheduled DH flight could also delay/cancel, and so it "shouldn't make any difference"--but I suspect it does....]