Professor Quirrell, in his absurd turban, was talking to a teacher with greasy black hair, a hooked nose, and sallow skin.
It happened very suddenly. The hook-nosed teacher looked past Quirrell's turban straight into Harry's eyes--and a sharp, hot pain shot across the scar on Harry's forehead.
"Ouch!" Harry clapped a hand to his head.
"What is it?" asked Percy.
"N-nothing."
The pain had gone as quickly as it had come. Harder to shake off was the feeling Harry had gotten from the teacher's look--a feeling that he didn't like Harry at all.
"Who's that teacher talking to Professor Quirrell?" he asked Percy.
"Oh, you know Quirrell already, do you? No wonder he's looking so nervous, that's Professor Snape. He teaches Potions, but he doesn't want to--everyone knows he's after Quirrell's job. Knows an awful lot about the Dark Arts, Snape."
Harry watched Snape for a while, but Snape didn't look at him again.
~Philosopher's Stone, Ch. 7 "The Sorting Hat"
Okay, so technically this is the first time we're introduced to Snape. He doesn't actually speak, but when you go back and re-read the series, you see a lot more meaning in it.
First, though, I'd like to know how Harry can tell that Snape's hair is greasy from across the Great Hall. For Harry to be close enough to see that Snape's hair is shiny (among other things that would tell you a person's hair is greasy) and deduce that it's also greasy, he'd have to be no less than twenty feet away. I don't think his seat at the Gryffindor table would be that close to the Staff Table.
Then, of course, this is the first time Snape's ever actually seen Harry in his years of teaching and waiting for the boy to come to Hogwarts so he could protect him. Snape's heart must have stopped when he saw how extraordinarily he looked like his father, and yet still had Lily's eyes. And right there, Snape looks at Harry's eyes, and he's probably struggling hard not to stare at them forever, because they're Lily's eyes, which he hasn't seen in at least thirteen years.
It sucks that Snape has to be portrayed as a potentially bad character so early on. :/
