OK, the whole Lily trying not to smile thing? That always bugs me, it's a memory. Memories are objective. Snape was feeling exposed and vulnerable and thinking the worst. Lily could have been smirking or so angry her lip was twitching or trying not to burp and spoil the stern effect she was going for. Also, the only reason SNAPE had any interest in Lily was because she was a witch. *mutter, mutter* The crazy is catching isn't it?