To clarify, they're not using James Corden's naked body as a canvas (that visual did initially spring to mind. And now it won't leave), but it's safe to say that all involved get more than they bargain for when their model emerges. Thankfully, there's something of a modesty screen to protect the audience from an extremely bare arse. Bare apart from the layer of downy hair.
As for the resulting works of art, well, they're something to behold. It helped that model 'Jonathan' had "absolutely no tan lines" - not that we get to see any of that, thanks the electric blue modesty bauble.
