As much as he would want to deny it, the things Severus didn't do said just as much about him as the things he did, to anyone who knew how to interpret behavior. Now, he would never have pegged Pansy Parkinson as a reader of body language and behavior, and he hadn't known her as well as other students of his, but he did know enough to not discount her to her face.
"No, I'm fine," he answered, sitting in the offered chair. A drink might have been nice, actually, and perhaps made him feel a little more at ease, but Severus Snape was still a paranoid man, and though he didn't actually suspect someone like Pansy to bear enough ill will toward him to do anything about it, he still was not eager to give anyone the opportunity.