Тыквик
Ага, оно самое. Третья книга.

“Lupin!” Snape called into the fire. “I want a word!”
Utterly bewildered, Harry stared at the fire. A large shape had
appeared in it, revolving very fast. Seconds later, Professor Lupin
was clambering out of the fireplace, brushing ash off his shabby
robes.
“You called, Severus?” said Lupin mildly.
14 глава и далее.
Кстати, в киноне это вообще отменно видно :)