The real Severus Snape
Severus Snape was never one for flights of fancy. So when the Headmaster had come up to him with saying 'I have a proposition for you,' he took off as fast as he could in the other direction. He had been lucky so far. Dumbledore had almost caught him coming out of the north tower. He had just finished visiting Trelawney to pick up his daily horoscope when he heard Dumbledore's unmistakable voice. Snape doubted anyone else in the Wizarding world had ever uttered the words: 'The Candyman can, because he mixes it with love and makes the world taste good.'
Hearing them come out of the old Headmaster's mouth was just plain weird. Someone else saying them would just be asking to be hospitalised. At least Dumbledore had an excuse: his apparent insanity. The song had the effect of a bell tied to a cat's collar. Snape usually had plenty of time to escape Dumbledore when he was in one of his particularly weird moods, always trying to set up the students or other such nonsense. Just the other day he'd been talking about the merits of teacher/student relationships and how he wouldn't mind at all if Snape indulged himself a little, hint hint. Snape poked his head out from behind the wall a little too early and Dumbledore started chasing him again. Snape ran all the way out of the castle before he managed to lose him again.
So it was no small coincidence that he was standing at the bottom of the Astronomy Tower looking up for any sign of Dumbledore when a massive blur came hurtling out of the window.