Does a “willing” creature know the outcome of the spell that is being cast on them?

Perhaps put another way, can you lie when casting a spell on a "willing creature"? For instance, I could cast Dimension Door on myself and one willing creature. I may tell him we’re going across that gorge, but in reality, we’re going 500 feet up for some tasty 20d6 fall damage for both of us, because why not?

Does a willing creature know what they are subjecting themselves to specifically? Or do they merely allow a spell to be cast on them, trusting that you’re going to do what you say you’re going to do?