Buffy belongs to Joss. Harry Potter is JK Rowling's. I just like to play in their sandboxes now and then.
Willow glared at the bare patch of wall. She’d been pacing in front of it for nearly half an hour now, and it was still as stubbornly door-less as it had been when she’d started.
“I don’t get it, Ginny. What am I doing wrong?”
Ginny Weasley shrugged.
“I don’t know. It’s usually dead easy to get it to appear. You just have to think of what you need the room to be.”
“I am thinking about it! Are you sure it still works?”
“Are you thinking about only that?” Ginny asked, sudden inspiration striking.
“Well… no,” Willow admitted. “I mean, I am thinking of what I need it to be, but I’m also thinking about how the enchantment on it works, and how difficult it was to integrate this with the other layers of magic in the castle, and what sort of dimensional magic they had to do to compensate for the room’s infinite possibilities, and-“
“Enough!” Ginny cut her off. She sighed. “Look, Willow, that’s great, but if you want the room to work, you have to focus on what you need. Not all the things you want to know. Let’s get you into the Room of Requirement first, and worry later about all the details of how it works.”
Willow nodded, looking slightly chastened.
“Ok. Think of something you need, and only that. Got it? Good. Now focus completely on that. Ready?”
Ginny stepped back out of the range of the enchantment and waited while Willow walked past it three times, visibly thinking extremely hard about whatever it was she’d decided she needed. Finally, a door appeared.
Willow let out an exclamation of triumph, and yanked the door open. She stepped into the room, looking delighted. Ginny hurried after her, shutting the door behind her. When she got inside, her jaw dropped. The room looked like a perfect replica of the Hogwarts Library except for two things. First, it was far more empty and pristine than the library had ever been. Secondly, all the signs Ginny could see on the shelves referred to different aspects of how the room might work.
“Like you said, get in the room first, then worry about how it works.”