I love Alan Rickman. J.K. Rowling told him Snape’s entire background story before they filmed the first movie so that he would understand Snape’s character, now watching these movies a second or third+ time is so great, because then you see strange or subtle little things Snape does, facial expressions, odd glances, ect, and you can be like “Thats why he did that!”

Like here, he sees Harry, and you can tell he is noticing how much he resembles his father, but there is no doubt that he also saw Lily’s eyes there too, then he notices that Harry’s scar is bothering him, and looks suspiciously at Quirrel because he knows he could be up to something. He already hates Harry so much because he’s a constant reminder of the love he lost and who he lost her to, and yet he tries to keep him safe from the first second he sees him. He doesn’t even have to think about it, he sees that something could put Lily’s son in danger and he instantly starts to put the pieces together to keep him safe.