The first time Fíli notices, he's not even sure what exactly it is he notices. In fact, he promptly forgets he's noticed anything at all when Bilbo sinks herself back down onto his cock, engulfing him in her warm, wet heat. He can only groan and grip her hips tighter when she again drags herself down and forward, like she wants him deeper, harder. Fíli wonders why they haven't had her ride him like this earlier before all he can focus on is the broken whimper and the exquisite tight clench of her around him as she circles her hips sinuously.
Later on, when he's sated and on the verge of sleep in his bedroll, he thinks about how hard and desperate she ground herself against him. But sleep claims him before he can think too hard on it.
The second time, he's almost too overwhelmed by her to notice. He spent all day hungry for her, wanting. It's sweet relief for him when he finally, discreetly steals her away the camp, lays her down on his coat, and buries his face between her lush thighs.
He's aware that he's maybe not so gentle as he should be, knows his beard is likely rasping harshly against her inner thighs as he moves his mouth against her, but he can't seem to help himself. He's only spurred on when Bilbo crosses her legs around his neck and uses her grip on the braids at his temples to hold him closer as he circles the tip of his tongue around her clit.
He drags the flat of his tongue lower, stiffening and curling it to fuck into her, to taste more of her. Bilbo uses the leverage of her legs over his shoulders and hands in his hair to push herself down against his mouth. Fíli nearly comes untouched at the shuddering gasp that spills from her lips as she circles her hips against his face, grinding herself along his nose. Fíli is immediately reminded of the other day, how she dragged her hips around desperately as she rode him. He tucks this thought away to think about later, diving fully back into his work, determined to have her make that sound for him again.
The third time he notices, he can't stop himself from saying something. His clever little hobbit had just saved all of them with her quick thinking about using the supply barrels to escape from the Elvish prison. Even though he is waterlogged and nearly sick from being stuck smelling nothing but apples for hours, he wastes almost no time in pulling Bilbo out of earshot to thank her properly.
He has her backed up against a tree and panting as he wickedly curls two fingers to press against that spot inside her that makes her legs tremble. He's using tongue, lips, and teeth to mark a red bruise on her collarbone when he feels her arch up into his hand and grind herself up into his palm. The moan she lets out is so utterly wanton that Fíli stops to look up. Bilbo's head is thrown back against the tree, closed eyes fluttering and parted lips glistening as more of those heady sounds spill from them.
He looks between them to watch as she arches up to rub herself on his hand, and his mouth goes dry. Realization hits him and he's harder than he's ever been in his life, cock aching in the confines of his trousers.
"You—you really enjoy that, don't you?" He can't keep the wonder from his voice and it startles Bilbo to opening her eyes. She catches his line of sight and flushes, staining her cheeks, chest, and the tips of her eyes a deep, lovely pink. She diverts her eyes down in apparent embarrassment.
In response, Fíli curls his fingers deeper and pushes his hand against her harder, causing her to gasp.
"Yes! Aulë, yes." She moans again and grinds back against his palm. Fíli looks back at her with wide eyes as heat pools low in his belly. He leans in to press his lips against her ear as he rubs his hand against her more insistently.
"Why? Tell me why." Bilbo shudders at the breath over her ear.
"S-so hard, feels so good," she pants out as she keeps dragging herself against his hand. "Love feeling you against me, feeling how strong you are." She raises her eyes, half-lidded and dark with lust, to meet his as she says this, and Fíli feels something inside him snap.
He slants his lips roughly over hers, running his tongue along her bottom lip as he frantically fucks his fingers into her, making sure to press his palm against her at each stroke. He thrills as Bilbo cries out and grips his shoulders tightly, grinding herself against him. He shoves his other hand down past his trousers and breeches and brings himself off messily in a few strokes as Bilbo arches and clenches against his fingers in her release.
Bilbo breaks the kiss, breathing shallow as she looks up at him in amazement. They only get a minute to catch their breath before they hear Thorin call his name. They look at each other for a moment longer before Bilbo turns a very familiar shade of pink, rights her bloomers and skirts, and scurries off towards the camp, though not without shooting a longing gaze back at the dwarf. Fíli is struck reeling by the look and what's just happened and takes a few minutes longer to put himself to rights before heading back.
Luckily for the company, they were hailed in Laketown as heroes and a sign of fairer times to come. They were accommodated richly: a feast thrown in their honor and each member of the company given their own room in which to spend the night.
Though the food was heavenly after months of little or none and the feasting merry, Fíli could not stop his mind from wandering back to earlier that afternoon. The image of Bilbo moaning and arched against his hand was burned into his retinas. He feels his cock start to harden as he recalled the way she looked at him, that he could bring her such pleasure in that way. He can still feel the circles she ground into his hand as she sought her climax, and suddenly, he has a plan.
He has to hassle Ori for one of his knitted scarves, but eventually Ori hands it over, though not before promising that he'd return it (and he would! Though perhaps a little worse for wear). As discreetly as he can manage, he sneaks into the building in which they will be spending the night and into Bilbo's room. With hands shaking and fumbling with excitement and nerves, he secures the scarf to one of the bedposts and makes a knotted loop at the other end through which he slides his wrists to bind them. Spread out and naked, he waits for Bilbo to come back to her room and find him.
When Bilbo finally comes into her room and notices him spread out on her bed, Fíli knows at her small, cut-off gasp that this all will more than make up for his troubles and the stiffness in his shoulders. He drinks in her face as she looks at him, a hunger flaring into life behind the surprise in her eyes. Fili just smiles at her wolfishly.
"I thought that you might like this, too. To have my body as you will, to commandeer my strength to your pleasure." Fíli watches the column of her throat as she swallows, and he's painfully hard under the heat of her stare.
Before he can fully register it, Bilbo is straddled across his hips and kissing him with a fierceness that surprises him. He responds in earnest, groaning as she pushes herself against him, her bloomers creating a delicious friction against his cock. She wastes little time in removing her waistcoat and blouse, all the while trailing her lips down to suck kisses into his neck.
Fíli is dizzy with arousal. He cries out in loss and arches up to try to follow her lips as she lifts herself off him. He tries to bite off a low moan as he watches Bilbo quickly shed her skirts and bloomers, but that soon becomes a futile task when she lowers herself back down onto him and her lips continue their assault.
He is reduced to pants and gasps as she trails her tongue and lips down the length of his torso, and he can feel the wet of her arousal as she slides her body down his thigh. His arms strain against the bind of the scarf as he arches towards the heat of the mouth hovering above his length. He yelps in surprise when Bilbo takes the whole of him into her mouth with no preamble. She begins to grind herself against his thigh, and he groans hoarsely at both the vibration of her moan around his cock and the feel of her moving and wet against his thigh.
Bilbo's tongue is wicked along his cock, and once he feels the tip of her tongue dip into the slit at the head, he knows he won't last much longer. Bilbo seems to be in the same position, as her hips move frantically against his thigh, grinding down harder and harder as she trembles against him. Bilbo bobs her head once more and Fíli is gone, moaning her name as he spills down her throat. Moments later, Bilbo shudders and then stills as she comes against his thigh.
They spend a few minutes panting and looking at each other as they did earlier, before Bilbo gingerly slides herself off of him and to his side to untie his wrists from the scarf. She looks down into her lap as her face reddens, but Fíli takes immediate advantage of his freed arms to fold her into an embrace. He pushes soft kisses into the ruffled curls of her hair, and finally Bilbo relaxes in his arms and lets out a small, contented sigh as she circles her arms around him in return.