Dumbledore and Snape are having a heated discussion in the bell tower. Snape is annoyed, wondering if Dumbledore has ever considered that Snape isn't really down with what's happening.
Wait, what is happening? What exactly is Dumbledore asking him to do?
Harry overhears the last part of their conversation as he comes to join Dumbledore. Snape runs into Harry on the stairs and gives him his signature Snape stare-down before leaving.
Dumbledore gets a little sentimental about how Harry has grown up, and lets Harry know this is going to be a super dangerous mission. He makes Harry promise that no matter what, he must obey the headmaster's commands, without question. Harry gives his word and they depart.