"Trust me," Charles says, and those are dangerous words coming from him, but Erik just shakes his head and arranges himself as Charles had requested, stark naked and stretched out prone with his arms folded under his head.
"Dare I ask what you're doing?" he murmurs as Charles rummages mysteriously where he can't see.
Charles sounds distracted when he answers: "Wanted to write a sonnet."
"Ah," Erik says, not at all enlightened. A sonnet. Of course. "And I'm your... muse?"
"No," Charles says slowly. "Well. Yes, I mean, of course you are, but not just."
Charles straddles his hips; he's still fully clothed, to Erik's disappointment, but Erik can still feel the shift of muscles as he settles himself. "Well, I was thinking..." He sounds a little unsure, almost shy, in a completely endearing way. "You could be my canvas."
All Erik can say is, "You are always full of surprises, Charles."
Charles just grins. "Thank you," he says cheerfully.
He uses a paintbrush that, despite fairly stiff bristles, still feels like it lends itself better to fluid calligraphy than to legibility. Erik closes his eyes, awareness distilling down to the prickle of brush sweeping against his skin, the cool wet trails that it leaves behind.
He tries to figure out from feel what exactly Charles is writing. A sonnet, he knows that much, but whose? Shakespeare, perhaps-- " 'Mine eye's due is thine outward part'," he quotes, " 'and my heart's right thine inward love of heart'?"
Charles pauses in his writing, and laughs. "No."
"Keats, then," Erik suggests. " 'O! let me have thee whole,–all–all–be mine! That shape, that fairness, that sweet' --"
"Stop talking," Charles says, still laughing, "you'll mess it all up."
He tries to imagine what he looks like, sweeps of dark ink on bare skin. "You know," he murmurs, "the others might notice."
Charles scoffs. "They'll never see it," he says. "Now, do I have to gag you?"
"Oh please do," Erik says, and Charles slaps the side of his ass and goes back to writing.
The first lines of the sonnet were broad stripes across his shoulders; later lines cramp up in the small of his back, and Charles sighs and does the last couplet sideways down Erik's legs, trailing down the backs of his thighs and the hollow of his knee.
"That tickles," Erik complains, when Charles sits back to admire his handiwork.
"You've endured worse," Charles says absently, and taps the handle of the paintbrush against Erik's ankle. "I think that will do, yes."
He can't see his back, but when he twists to try to see the back of his leg, hoping to glimpse a word or two that will give him some clue, all he can see is skin. The sight jars with the feel of still-drying ink, and he looks up at Charles in confusion. "There's nothing there."
Charles just raised an eyebrow, radiating innocence. "I never said I was using ink," he pointed out, "or paint, or anything else of that sort."
"Roll over," Charles says, and nudges his hip. "You won't stain the sheets, I promise."
Erik rolls, but sits up as he does, one hand tangling in Charles's hair. "What did you use?"
"Just water," Charles says, showing the paintbrush, still wet but perfectly clean. "It seemed more permanent than anything else."
The skin of Erik's back is dry now, and he still has no idea what Charles wrote -- it could have been anyone's work, even a sonnet of Charles's own creation -- but he can still feel the essence there, and it is enough.
He kisses Charles, and then murmurs against his skin, " 'But love me for love's sake, that evermore thou may'st love on, through love's eternity.' "
"Something like that," Charles says.