The apartment that is given to Gretta (Keira Knightley) and Dave (Adam Levine) in New York City is the real house of Levine and his wife, Behati Prinsloo.
Keira Knightley had to learn play guitar for the role. Her husband, musician James Righton, offered his help. According to Knightley, his lessons were disastrous, and almost led to "divorce and murder".
Mark Ruffalo and Keira Knightley spent a couple of weeks hanging out with each other before production began, to build their chemistry and their characters, and to work on their improvisations.
The red electric guitar Adam Levine (Dave) plays at the end of the movie is his actual guitar that he plays at concert; it's his personal favorite.
Sharman Macdonald: A customer seated behind Gretta (Keira Knightley) and Dan (Mark Ruffalo) in a small café, just before Dan storms out.