The boy-turned-Titan before Rivaille seemed to be thinking hard, almost in a trance. Emerald eyes with irises as big around as wagon wheels watched the Corporal of the Survey Corps intently as he obsessively cleaned his 3-D Maneuver Gear. The 50-foot humanoid sat Indian-style across from him in the large warehouse, most of his frame hidden from view by the shadows circling the small ring of light cast by the lantern.
"Got something on your mind, brat?"
The Titan's head lifted slightly in surprise, its eyes widening slightly. He rumbled quietly and looked away from Rivaille, as if embarrassed. Skull-like jaws parted to release a small cloud of steam and the Titan's brow furrowed deeply. He slowly reached forward with one hand, his little finger outstretched. Rivaille watched him, not making a movement or sound. Whatever the Titan planned to do, he'd be ready to move out of the way if needed. He didn't fear the Titan shifter, never had, but he knew that one stray move could injure him, and if Eren had lost control, he'd have to act fast.
His caution wasn't needed, however, for the next thing Rivaille knew the Titan's little finger had gently brushed against his adopted sister's cheek, moving a lock of hair from her face. Rivaille was impressed by the amount of control the kid demonstrated over such a massive frame, able to perform such a task without harming anything. He raised a brow as Eren looked back to him.
"Pretty good..." He saw the Titan's eyes light up with happiness at the veiled praise and felt more than heard a happy rumble drift through the warehouse.
"Not so loud, bête! Do you want to wake up the entire squad?!" he hissed, and Eren's jaw quietly snapped shut on his purring, his back straightening a little.
"Anyway, I guess you've been working at being less destructive?" Eren nodded, rumbling an assent more quietly than before.
"Well, I guess it's paying off, brat. Maybe things will actually stay clean when I get you and the squad to clean them, now..."
Eren let out a small chuff of a laugh, surprising Rivaille.
"Was that funny to you, brat?"
Eren straightened and shook his head, his constantly-messy hair swinging back and forth slightly. It didn't escape Rivaille's gaze, nor did the shine of oil that the lantern produced on it.
"Speaking of things being cleaned, you desperately need a bath..."
Eren tilted his head to one side, looking confused. As he did, a lock of his hair fell more completely into the light. Rivaille stood, grabbing it and tugging lightly.
"Look at this...filthy. Your hair is so oily it looks like glass, brat!" Eren's jaws parted in surprise, teeth the size of small children drifting apart slightly as a bit of steam lazily trailed between the rows. "Tomorrow morning, if you're still stuck in this form, we're making sure you get clean, first thing. I refuse to be stuck with a brat that can't even keep himself clean..."

The next morning Eren was woke by an insistent tug on one of his unkempt locks, opening his eyes to spot Rivaille standing right in front of his nose.
"Come on, brat. Clean up time."
Rivaille escorted him and a few of the others from the squad - including a rather overexcited Hanji - to the nearest river, each bringing along a bucket full of cleaning supplies.
"What are we doing again?" Mikasa asked quietly, unsure if she'd understood the Corporal the first time he'd explained the situation. Or the second.
"Watch. Eren!"
The Titan's attention snapped to him.
"In. Now." He pointed to the river. After a moment's pause, the Titan obeyed, stepping into the river. He knelt so Rivaille and the others were easier to see, looking just as confused as everyone else. Rivaille then grabbed one of the buckets and waded into the river, standing a short distance from Eren. "Get your hair wet."
The Titan looked at him in confusion, and he gave the much-larger figure a pointed glare. After a second he bent and dipped his head in the water. Rivaille had a handful of shampoo waiting and viciously set to scrubbing the stubborn, perpetually-unkempt locks. Eren blinked in surprise, uttering what sounded like a yelp, but instead of pulling back he froze, not wanting to send Rivaille flying - the Corporal would've probably massacred him for it.
After a moment he realized - the scrubbing may have felt unfamiliar to the Titan body, but it also felt...really good. After a short pause he started to slowly lean his head in the direction Rivaille stood almost up to his shoulders in the river water. It looked as if he was only moving to give Rivaille access to the top of his head...at least until Hanji started bouncing and squealing about him making some kind of noise. They listened, and - sure enough, Eren was practically whining as he leaned his head into Rivaille's hands, obviously enjoying getting his scalp rubbed.
"Well, brats? What are you waiting for?! I didn't drag you all out here just to have you stand there and watch!"

Optional extra bit:

Pretty soon Eren was practically laying down along the river, his head resting on the bank as they worked to scrub the rest of him clean. Rivaille was particularly ruthless, often getting a few warbling growls in response to whatever it was he was doing.
Armin, Mikasa, and Hanji were the only others that didn't seem extremely wary of the Titan shifter, the young blonde managing to get Eren to reproduce one of his small whines as he scrubbed underneath his jaw and behind one ear. He laughed slightly as Eren leaned towards him like a kitten, practically begging for more.