His writing habits had thrown her off, at first.
Castle could literally be in the middle of a thought when inspiration struck, and he would always dash off to scribble it down somewhere.
He even did it at night. Barely half-two in the morning, and he would bolt upright next to her mumbling nonsense—and then disappear to his office to write furiously until dawn broke.
Hell, he had gotten inspired after their first time together, leaping out of the bed to stride bare-assed for his laptop.
It frightened her a bit in the beginning. It had been emotionally vulnerable enough for her to confess to the lie that almost ripped them apart, and then he was leaping out of bed post-coital to disappear God knew where to write.
She eventually learned the pattern behind it. After the first few times, she even grew brave enough to venture to the office where she would perch precariously on the edge of his desk, watching him write.
Soon, it became almost the reverse of their partnership in the 12th. Castle would sit and bounce ideas off of her, and Kate would sit and watch the man she loved do what he loved.
And in the end, he learned to keep a notebook by the bedside—keeping both of his loves close at hand.