What would the consequences be if I did not do (whatever you just mentioned)? Implies that the speaker either believes they are right not to do something, or else they are suspicious of the other person questioning them about it. A: "I'm warning you for the last time to take that money to the bank and deposit it!" B: "And supposing I don't? What exactly are you going to—report me to police?" A: "We need to report those earnings so we can pay taxes on them later." B: "Supposing we don't, though? Would anyone ever find out about it?"