Prompt: 2x23 AU. Beckett catches Castle staring at her when she kissed Demming.

Kate Beckett was the only person who could leave Richard Castle speechless.

Like now. Seeing her kissing that guy from Robbery — Demming, with the long chin and the squinty eyes. Castle found himself frozen in place, his heart stuck in his throat. Something welled up inside him, an emotion he didn't recognize and couldn't place.

He didn't like it, he knew that much.

On second thought, he wasn't quite speechless. Castle had the words he wanted so desperately to scream in mind. But the shock kept his mouth clamped shut, his feet glued to the floor despite his internal cries to run.

As her lips locked with Tom Demming's, Kate Beckett felt her eyes flutter open again. Demming broke the kiss and gave her hand a squeeze before he left for the night, and she caught something out of the corner of her eye.

Kate watched Demming round the corner before looking to her left, her heart skipping a beat when she saw Castle standing there. More importantly, she saw the look on his face.

Crap…

She drew a ragged breath, wondering why her hands were suddenly shaking. Kate turned to Castle, her mouth open. No words came, though, and before she could react, the writer turned to leave.

Castle had to get out of there. He couldn't be at the Tweflth anymore.

He jammed his finger against the button to summon the elevator as hard and fast as he could, desperate to hear the ding of its arrival. His heart pounded in his chest, his jaw clenched. He felt something welling in his eyes, burning.

Hurried footsteps approached from behind, just as the doors to elevator swung open. He walked into the elevator, keeping his back to the doors. He just wanted them to shut and for the box to send him on his way.

This wasn't his place anymore.

The doors weren't closing, though, and the thud from behind told Castle that someone had grabbed the doors to keep them from latching. Castle cursed under his breath, his shoulders tensing again.

"Castle…"

Kate stepped into the elevator, letting the doors close behind her. Truth be told, she had no idea what she was about to say. But she knew the look on Castle's face just moments ago, and though she'd had her suspicions in recent months, seeing how dark and cloudy his blue eyes had become just…hit her hard.

"Castle, look at me."

For several seconds, the man across from her didn't turn around. She saw the way his blazer hunched at his shoulders, and when she looked down, she saw that his hands had balled into fists.

"Castle…"

"How long?"

Her brow furrowed. "Excuse me?"

Slowly, Castle turned to face Kate. She hadn't seen the look on his face before, and she hoped she never had to see it again. He looked like a wounded animal, a swirl of hurt and anger written all over his — admittedly handsome — face.

"How long have you and Demming been seeing each other?"

"Um…not long." She frowned and swallowed. The elevator had started its journey to the ground floor, and without thinking, she reached behind her to stop the car. "That a problem?"

"No." Castle shrugged, purposefully avoiding Kate's gaze.

"Then why are you acting like it is?"

He sighed, but there was no answer. Kate folded her arms over her chest, annoyance and anger etched onto her face, when the reality was, her insides were far more twisted in guilt and self-disappointment. That surprised her, and she couldn't place where it came from.

Or maybe she could, and she just didn't want to admit it.

"I don't know, Kate." He sighed again, leaning against the back wall of the elevator. "I guess I just thought maybe—"

"That what?" She spat. "That I could still be one of your conquests?"

"No!" Castle pushed himself off the wall, his jaw set. "God, Kate, is that what you—" He shook his head, pacing back and forth. "That's not me anymore. That hasn't been me for a long time, Kate. You know why?"

Tearing away her gaze, Kate shook her head.

"For one, living like that was boring." Castle sighed. "Secondly…Kate, do you really mean to tell me that you haven't noticed anything between us lately?"

"You mean other than the fact that I only want to shoot you half the time, as opposed to all the time?"

"Come on, Kate." He shook his head. "You saved my life when you shot Coonan. I saved yours when I pulled you out of your apartment."

"So because we've saved each other's lives, I'm not allowed to date other people?"

"No, Kate, that's not…" He sighed again, leaning back against the wall of the elevator. For someone who made a living with words, he had a hell of a time with them when they mattered.

"I like you, okay?" He scoffed, shaking his head. "There. I said it. I like you, Kate."

Kate shook her head. "I saw the look on your face, Castle. That wasn't like."

"Fine." Castle's jaw clenched again. "It's more than that. But don't worry; I'll get out of your hair, leave you to be with Demming."

Kate frowned. "What are you—?"

"We've worked our last case, Detective." Castle reached across to get the elevator started again, keeping his gaze on the ceiling. "It's over. I'm done."

Kate slammed the button again to make the elevator stop before turning back to regard Castle again. She pushed him back against the wall, gritting her teeth and jabbing a finger into his chest. "So…what? You don't get your way, so you take your ball and go home?"

"It's more complicated than that, Kate."

"No, it's not."

Before Castle could react, she grabbed the lapels of his blazer and kissed him. The kiss was one-sided at first, Castle too dumbfounded to react, but soon his hands rested on her shoulders and his lips found a steady rhythm with hers.

Once their lips parted, Castle huffed a ragged breath, cocking his head to the side. "Kate…"

"No." She tugged on his lapels again. "We're going to go to Remy's, have dinner, then hit a bar for some drinks. Tomorrow morning, you're going to come back here, two cups of coffee in-hand, and you're going to pull on my pigtails some more."

A sly grin crept onto the writer's face. "And what if I want to…actually pull on your pigtails?"

She pushed the button again, turning her back so he wouldn't see the smile on her own face. "Let me break things off with Demming first. Then we'll talk."