A/N: I have only the vaguest idea where this came from (although I do, fortunately, know where it's going). I was rereading Heat Rises and Nikki's discovery that Rook is a secret romance author made me ask myself: What if Castle had a secret too? What if, in a moment of need, he wrote some steamy RPF fan fiction? And what if Kate read it? And then they both found out what each other was up to - would sexy!times ensue? Of course the answer is yes. And with that in mind, I just want to tell everyone, this story is currently rated T in the interest of keeping it out in the main sections of ff..net, but that rating will eventually change to M. Don't say I didn't warn you. :-)

And, as always, a disclaimer. I don't own this show, or the books, or any of the characters that are familiar to you. If I did, the whole shebang would be far less PG (i.e. they'd be fucking like bunnies already). So, Andrew Marlowe, could you please take the hint already. We want some action. Soon.

Castle's hand curled into a fist as he sat staring at his computer screen. His writing was not going well tonight. Actually, writing had not been going well for several nights, ever since he and Beckett had concluded their last case.

It wasn't the case itself that was the problem, seeing as it was a fairly mundane murder that occurred during an ill timed breaking and entering by a perp who was kind enough to leave behind plenty of identifying evidence. The only interesting part had been when they went to apprehend their inept suspect at his messy apartment.

The not-quite-a-criminal-mastermind had first let them in with a show of innocence and then tried to evade them by running straight for the door, despite the fact that his path led right through Beckett. Not a tactic that would have worked in most normal circumstances, but the guy slipped on a filthy tee shirt and careened into Kate with no warning. Like a stack of dominos, they all went down: Beckett, the perp, and knocked off balance by someone's flailing arm, Castle on the bottom of the pile. The perp scrambled out the door on his knees only to be apprehended by Esposito and Ryan before he even made it to the stairs.

Back in the apartment, Castle was still lying on the floor, moaning as he attempted to get his breath back, and vaguely realizing that the tumble had left Beckett sprawled on top of him.

He moaned a bit louder.

"Are you okay Castle?"

He could hear the note of concern in her voice and let out another vague gasp. She shifted so she could face him, concern in her eyes.

"Is it your head?" she asked, her fingers gently exploring his scalp.

The contact, coupled with the fact that she was still sprawled across his chest, her face only inches from his, was more than he could take and another low noise escaped him. It must have sounded different, or at least prompted by a less painful sort of distress, because Beckett's eyes narrowed and her grip tightened in his hair. She swung herself across him until she was straddling his waist … well, not exactly his waist. He was acutely aware that she was actually somewhere south of his waist.

"If you're faking I swear I will injure you myself," she said with a glare.

Unfortunately, Castle was completely unable to answer since his entire being was focused on the rather inappropriate response her proximity was causing. A response that apparently made her notice their relative positions since she promptly released her grip on his scalp, causing his head to bounce against the floor for the second time that day, and leapt to her feet as if she had been scalded.

"Castle," he heard her mutter in disgust as she strode out the door to assist the other detectives. At least he thought that was what she said. It might have been another, somewhat less appropriate word.

Then the incident was over, apparently forgotten, at least by Beckett. Castle also shrugged it off while he was at the precinct, but that all changed when he was at home and writing. It was hard enough to put himself in his characters lives when their fictional relationship had gone so much further than the relationship that was the focus of his own life. It had gotten even harder now that he and Kate had grown closer without ever reaching where he wanted to go, but he usually managed it. So why had that one little moment had completely wrecked his concentration? Perhaps it was the fact that his chivalrous vow to wait for Kate meant that he hadn't had sex anytime in recent history. And no, self service didn't count.

The thing was, whenever he sat down and pictured Nikki Heat, all he could imagine was the feel of Kate's thighs as she straddled him, and the delicious heat of her … damn it, this was getting him nowhere. He had even tried his usual standby for this sort of situation and skipped ahead to write a planned sex scene. Unfortunately, that hadn't been any more fruitful since the material he found himself writing was far too steamy to be publishable in any mainstream novel.

That was when the idea struck him. Maybe what he needed was to go ahead and live out this fantasy. Okay, not in real life, that would most likely end with Beckett inflicting some sort of grievous bodily harm. And completely apart from a fondness for retaining all his body parts, he really had no desire to see her arrested for assault, even if he did find the picture of her in handcuffs quite exciting. No, he would have to live it out in fiction … fanfiction to be exact.

Even though he knew he shouldn't, he had ventured over to the fan sites several times and read some of the material that his, and Nikki's, obsessed followers had come up with. Some of it was surprisingly good. There was also the fact that much of it was very explicit, and none more so than the stories that fell into the category of RPF, otherwise known as real person fiction. It seemed that some fans were not satisfied with writing about Nikki and Jameson but went the extra mile to write about Castle and Beckett themselves. He found the stories oddly tantalizing, if also vaguely creepy. Right now, however, it was just the venue he needed.

He, Richard Castle, was going to write a fanfic … an RPF fanfic … a very, very sexy RPF fanfic. And no one would ever know.

Flexing his fingers over his keyboard, he closed his eyes for a moment and let his imagination take him wherever it wanted. Then he shut out the real world and started to type.

Kate Beckett was far from oblivious to the longing looks that her faithful sidekick, Richard Castle, was prone to sending in her direction any time he thought she wasn't looking …

God, that made him sound so pathetic, but when writing fanfic it was probably best to stick to the usual tropes. He certainly didn't stare like that in real life. Well, he didn't do it very often. Okay, he only did it when he was sure she wouldn't notice. Anyway …

Although she would die before revealing it to him, she actually liked the looks. They warmed her up in a way that Josh never had. Of course, that was exactly why she had to hide her reaction, a relationship with Castle would be sure to upset their fragile equilibrium and their friendship was too important to risk in that way. Then again, she had read those truly hot scenes in his books … and imagined reconsidering her position. Of course every time she started thinking this way, Castle did something that was so ... well, so Castle, that she promptly gave herself a mental slap and stuffed any fantasies firmly back into the furthest crevices of her mind.

Just then, the subject of her morning musings strutted into the squad room and sprawled unceremoniously in his usual spot beside her desk. She forced a stern look onto her face. "Getting a late start today, Castle?" she asked.

"No, an early one actually. I was out at the crack of dawn to pick up the newest Blue Avenger comic. See," he said as he held out his copy, still smelling faintly of the printing press.

"Very nice, but couldn't you wait until later to get it. Somehow I doubt they will run out."

"Of the comic, no, but the first twenty purchasers also got this," he said proudly as he spread open his shirt to reveal a stretchy Blue Avenger costume top.

"Underoos?" she asked scathingly to cover the fact that the form-fitting spandex outlined the shape of his chest in a rather enticing way, if you ignored the fact that he was wearing a silly superhero costume under his street clothes …

Castle stopped typing for a moment, distracted by the thought of actually getting a really cool Blue Avenger suit. Then he ran a hand across his own shirt, distracted even further as he pictured Beckett giving him a heat filled glance. He shook his head to clear his mind and turned back to his computer ...

Castle was undaunted by her disdain and turned to display his newest acquisition to Ryan and Esposito, who gave him an encouraging thumbs up.

"Very cool, bro," Esposito contributed.

"Almost looks like the real thing," Ryan told him.

Smiling, he looked back to Beckett. "I may not be a cop, but at least now I can be a semi-authentic superhero."

His enthusiasm was infectious and she couldn't help but smile back. "Just make sure you remember that unlike the Avenger, you can't dodge bullets at superhuman speed."

"I will ... if you will," he told her, his look suddenly serious.

She felt herself drowning in his gaze and had to tear her eyes away before she could answer. "I'm pretty sure I learned that lesson the hard way."

Who knows how long they might have gone on staring at each other, but Esposito broke the moment when he put down his phone and announced that they had a body.

Beckett gave herself a mental shake and looked up. "Details?"

The words were flowing now, and Castle wrote quickly, anxious to get to the good parts. He would have happily gone there immediately. After all, porn-without-plot, otherwise known as PWP, was an accepted genre in the world of fanfiction. He had once read a truly wonderful piece of smut that took place entirely in a broom closet and then spent the next week twitching every time he saw janitorial supplies. However, his own writerly instincts were too honed for him to avoid the pesky nuisance of creating an actual story.

Fortunately, years of practice had enabled him to write rather rapidly, and the details of the case slid past quickly. A little gore, a bit of forensics, a few red herrings ... and Castle looked up to see that his desk clock read two AM. As much as he hated to stop when he was on a roll, he could feel a yawn trying to rise from his throat. Even though he had not gotten to any of the steamy parts, the very act of writing this story, using their real names, had left him feeling strangely purged. Besides, he already had more than enough material to begin posting. All he needed now was a suitable pen name.

What to use? Something with his initials? No, that was too boring. He toyed for a moment with ReallyRick, but discarded that as too obvious, especially if anyone from his publisher or publicist wandered over to check out the fandom. They might recognize his writing style and become suspicious. Wait, he had it ... Alteredego. It was perfect - meaningful, but also just the sort of cryptic pseudonym that an ambitious fanfic writer might adopt.

A few clicks later, he was the proud owner of a new literary profile with a freshly posted work in progress. Now all he had to do was sit back and wait for the reviews to roll in. Even though he was a New York Times bestselling author, or maybe even because of it, he knew he would spend the next few days obsessively checking his email for review alerts. No one ever got too famous to enjoy a little positive feedback, even if these comments were likely to be rife with capslock and replete with internet acronyms.

Smiling to himself, Castle clicked the button that would forward reviews to his smartphone, and then, with a huge yawn, he headed off to his bed … and what he was sure would be pleasant dreams