i just think Alan nailed all the nuance of who Snape was. If i had to pick he would be one of the actors who really brought a deeply conflicted character to life without making him a cartoon. It would have been easy to mess Snape up, but he walked that line. While he was mean and unfair it was never the twisted cruelty of the death eaters or the government. And he always kept enough mystery that which ever side he fell on it would have worked.
But the hints were there if the viewer paid attention.
By master class i would point to his work and tell young actors to use that as a goal when playing a decade spanning role. His was a far harder role than say the Ironman part even thought it was the anchor for the mcu franchise.