Svaren finns ganska mycket i The Deathly Hallows , The Lost Diadem-kapitlet.
Rummet måste vara tomt för en person för att helt kunna förändras. Eller snarare, så att rummet kan återställas.
Rummet behöver dock inte vara tomt för att kunna förändras för de personer som redan är inuti det. De behöver bara vara specifika med vad de vill att rummet ska vara.
Så det behöver inte förutsäga framtiden, det behöver bara upptäcka en stark, specifik önskan för att skapa något. Harry's whistle på hyllan skapades på samma sätt som Neville gjorde en dörr till Hog's Head, och hur DA utvidgade rummet.
Den relevanta delen av boken:
‘Where are we?’
‘Room of Requirement, of course!’ said Neville. ‘Surpassed itself, hasn’t it? The Carrows were chasing me, and I knew I had just one chance for a hideout: I managed to get through the door and this is what I found! Well, it wasn’t exactly like this when I arrived, it was a load smaller, there was only one hammock and just Gryffindor hangings. But it’s expanded as more and more of the DA have arrived.’
‘And the Carrows can’t get in?’ asked Harry, looking around for the door.
‘No,’ said Seamus Finnigan, whom Harry had not recognised until he spoke: Seamus’s face was bruised and puffy. ‘It’s a proper hideout, as long as one of us stays in here, they can’t get at us, the door won’t open. It’s all down to Neville. He really gets this Room.
You’ve got to ask it for exactly what you need – like, “I don’t want any Carrow supporters to be able to get in” – and it’ll do it for you! You’ve just got to make sure you close the loopholes! Neville’s the man!’
‘It’s quite straightforward, really,’ said Neville modestly. ‘I’d been in here about a day and a half, and getting really hungry, and wishing I could get something to eat, and that’s when the passage to the Hog’s Head opened up. I went through it and met Aberforth. He’s been providing us with food, because for some reason, that’s the one thing the Room doesn’t really do.’