There is never anything good following the phrase "I need you to" on a plane. Typically I hear it from self entitled parents who want to force a seat swap.."I am going to need you to swap seats with my husband!" kind of thing. I never swap of course, but I can refuse politely if asked politely. If the request starts right out as an order, they get treated appropriately. And the same applies to FA's asking me to do something THEY are supposed to do.