Casual setting, amazing food. Dietro l'angolo is probably a unique set up for many Americans, so…read morelet me try and get you on the right track. Dietro is both a restaurant and a deli of sorts, and you order at the counter (the food will be brought to you, but they'll give you your beer or wine straight away at the counter).
They have several fresh pastas available every day (check the sign on the wall for your options) and you can pick what type of sauce you'd like to pair with the noodle. You can also order whatever side dishes they've got in the case to go with your meal (see my lunch photo), so if you want a bit of pasta with some veg, go ahead and order it! They'll get everything hot and delicious and bring it out for you.
Gippy, who runs the place with his brother, is incredibly gregarious and great for a conversation if you speak Italian. And if not, at least great for a few words of exchange. There's both outdoor and indoor seating (mind that the tables are for groups only, and there are communal benches for singles or doubles) available.