I'm curious about the difference in nuance between "You're smoking again?" and "Are you smoking again?".
"You're smoking again?" sounds more casual than "are you smoking again?". This is because it takes a statement and turns it into a question without using any question words or phrases. So the difference in nuance is that it sounds effortless and more informal. Here's another example of this: Ex: Do you want something to drink? Ex: Want something to drink?