The longer the man stared at him, the more curious the Doctor became. "Take a picture, it'll last longer," he quipped, but his smile faded too quickly. There was something about this man that was different, and it only took a little effort of concentration on his part to reach out, and what he perceived took him aback. There was more difference in Castiel than what he initially imagined, and he was about to speak the words, What are you? when Lydia spoke.
Banshee?
Angel?
Maybe that's what they called their respective races, but the Doctor had too much experience traveling the universe, encountering what people thought were gods, vampires, ghosts, but all of them had a scientific explanation.
At this moment, what did it matter? Both Lydia and Castiel looked like they were having a good time, so why did he need to be a party pooper?
"Give that angel a marshmallow," he told Lydia with the same enthusiasm as a game show host announcing what prize was just won. "You've never had a fizzy drink? Wait right here... I'll be right back."
With those words, he trotted back over to the TARDIS and went inside.