The first sound always comes quietly, strangled, as Loki tries but fails to kill it in his throat. It makes Thor burn, his need redoubling instantly at this first proof, this first choked thread of truth.
Everyone else, all those who call Loki the Liesmith, they think that Loki must detest the truth, and so he tortures it and ties it in snarls until the half-truth has become a whole lie. Thor knows they are wrong. To Loki, the truth is too precious to share, and so he hoards all his truths jealously and deigns only to give others the counterfeit imitation of them.
Loki doesn't even give Thor the truth-- not until Thor forces him to, anyway.
Thor opens his mouth wider and slides down farther on Loki's length, down until the head of it rests snugly at the back of his mouth. Loki permits him another piece of the truth, another genuine shred of sound and need and pleading. Thor rewards it with a long suck, working his tongue along the underside of Loki's manhood.
His tongue is not silver, but then again, he isn't trying to lie.
Slowly Thor slides up and down, being patient the way only Loki can make him be; if he rushes, he will wring the pleasure out of them both long afore it has broken Loki's walls at all. He draws off, slurping over-playfully at the crown of Loki's sex before he remembers that he must, must take this seriously. For all his supposed sense of humour, Loki cannot take a jest, not even the lightest teasing about his switch-thin body or faltering swordwork; when he feels he is being made fun of, he closes himself off immediately. Thor doesn't understand how his brother can profess to feel so alienated from Thor and his friends when they take every opportunity to rib him as comrades do, but Loki, he has learned, is a creature of great (and unnecessary) complexity.
Somewhere above Thor, Loki makes a strangled moan, the sound throbbing in the near-silence of the bedroom. Thor licks him again in reward, laps at the big vein on the curved underside of Loki's length.
It is a very nice member, Thor once tried to tell Loki, something to be proud of, and not at all what one would expect from looking at Loki with clothes on-- and then his brother had stormed off and not spoken to Thor for a month. He suspects Loki still remembers this (as he remembers and hoards every truth) and so now Thor can't even attempt to say anything about Loki's cock, lest it make Loki recall that other incident.
Loki makes a more urgent sound, distracting Thor from the wickedness he was doing with the tip of his tongue. Flat on his back on the mattress with his hands roped to the headboard, Loki rolls his head against the pillow to look down at Thor. His eyes are wild and the gag between his lips is dark and wet. Spittle shines on his chin, and for this Loki looks perfectly furious even as his hips twitch in need.
Thor mutes his laugh of delight at the sight, because he cannot be seen to jest. Instead he goes back to work, curling his tongue to catch the thin droplets of fluid beading from Loki's cock. Loki yelps again, the sound harsh and dulled through the gag.
Some would think that gagging Loki's silver tongue is not the way to make him be honest. Thor knows that it is exactly the way to make Loki be honest: the gag keeps him from lying, and then Thor's tongue coaxes the truth out of him. It's hardly an articulate truth, those whines and cries and gasps, but it hardly needs to be. It only needs to be the kind of truth Thor understands, raw and uncomplicated and unfettered with all the subtleties that Loki usually puts on his words to confuse Thor.
Soon enough Loki groans, and then begins to whimper, falling apart piece by piece as he realises, yet again (and Thor will teach Loki this truth as many times as he must), that he can let his voice run free because the gag will keep him from spilling any secrets that might embarrass him. Loki's control over his body rattles apart at the same pace, leaving him shaking and twitching uncontrollably as Thor cradles the crown of Loki's cock on his tongue and sucks lazily.
Here where Loki cannot hide anything from him, Thor knows exactly what his brother is thinking and feeling. Loki is close. He sits up and takes Loki's spit-slicked length in his callused palm, straddling Loki's thighs as Loki tries to give a frantic buck of frustration at the loss. Looking down at Loki, Thor finds himself suddenly glad that his brother doesn't share his truths with the world after all.
Loki has taken the gag between his teeth, biting on it hard enough to leave red marks at the corner of his wet, shining mouth. His teeth gleam; his throat works and flexes as little, desperate gasps are forced from it. The sinews in his arms twist and writhe beneath the skin as Loki pulls at the ropes, and the tight muscles of his biceps cord with effort-- and truly, they don't seem so weak after all, even if they are woefully lean. Sweat gleams on Loki's face, lending a manic brightness to his begging eyes.
"Easy, brother," Thor coaxes, slow-curling his thumb over Loki's slit and then sliding his hand back down, working so very gently with his rough hands.
Loki shuts his eyes tightly and whimpers, an escalating series of noises that climb higher and higher in his throat.
"Come along, easy... it's all well, Loki, you can--"
And with no more than that, Loki finally releases in short, violent bursts over Thor's fingers, shaking so hard that it's as if his body threatens to come apart entirely. Harsh sounds break from between his clenched teeth, long visceral cries torn from the very depths of his chest that end in half-sobbing gulps and deep, frantic gasps for air. Through it all, Loki strains and shudders so hard that Thor knows, he knows Loki has to be screaming out a far greater secret than simply I love you, but it's so guttural and tangled up in pathos that he doesn't know what.
After the eternity of six heartbeats, Loki falls slack and quiet, wrung to total exhaustion. The only sound is the fast, wet suck of breath through the sodden gag. Thor's hands shake as he leans forward and picks at the knot of Loki's gag with slick fingers.
Loki blinks up at him blurrily, his acute eyes still gentled, muzzy. In these moments he reminds Thor of a younger child, nearly a baby, his sleep-heavy body curled up in Thor's arms in their shared bed and blinking uncomplicated love at him. Still, in the here and now Loki is himself enough to smile like the welcoming curve of a sickle when Thor moves up to straddle Loki's shoulders, still shaking and perhaps clumsy in his eagerness as he settles his brawny thighs on either side of Loki's head.
"Aye," Thor moans, thumbing Loki's swollen lower lip and guiding the head of his cock into Loki's mouth. Loki swallows him down easily, silver tongue skating across the shaft and making Thor groan loudly, his truths freely given.
Thor has the abrupt sense that Loki is suddenly far more in control, even with Thor's cock sliding thick and heavy between his lips. He grasps at the vague thought, trying to make sense of it, but it slips away as Loki's teeth scrape exquisitely along his length.
Loki will be gagged a little longer, Thor tells himself, and shoves the rest of the suspicion away in order to surrender to the mounting pleasure. He cannot possibly be lying just yet.