As respostas são encontradas em The Deathly Hallows , no capítulo The Lost Diadem.
A sala precisa estar vazia de uma pessoa para mudar completamente. Ou melhor, para permitir que a sala seja redefinida.
No entanto, a sala não precisa estar vazia para mudar para as pessoas que já estão dentro dela. Eles só precisam ser específicos com o que eles gostariam que o quarto fosse.
Portanto, não é necessário prever o futuro, basta detectar um desejo strong e específico para criar algo. O apito de Harry na prateleira foi criado da mesma forma que Neville fez uma porta para Hog's Head, e como o DA expandiu a sala.
A seção relevante do livro:
‘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.’