Severus tensed, his back stiffening and his fingers pulling just a little tighter in her hair. it had been unexpected, to say the least, and for a moment he wasn't sure what he should do.
But that passed very quickly. His hand that had been clenched into the upholstery of the sofa - he hadn't even realized until now that he'd been gripping it so tightly - came to rest at the side of her face, slipping behind her jaw eagerly, almost possessively. He still had the presence of mind to feel clumsy as he kissed her back, his teeth scraping gently across her lower lip - it had been a very, very long time - but she was giving him more than enough to follow. Amid the heated, urgent, confused outpouring of feeling there was definitely gratitude.