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".
Dan's last name is Mulligan. In golf, a mulligan is a free shot, or a second chance, for a golfer to replay his or her last swing. The surname choice may be a reference Dan's "second chance" at having a successful career.
In May 2016, John Carney told the Independent, that while he considered Mark Ruffalo a great actor, and Adam Levine a joy, with whom to work, he didn't like Keira Knightley's performance in this film under his direction. Carney said "It's not like I hate the Hollywood thing, but I like to work with curious, proper film actors, as opposed to movie stars", and "I learned that I'll never make a film with supermodels again." He later apologized and recanted his statement.
When Dan and Gretta release her debut album online, her full name on the album cover is revealed to be "Gretta James", which takes inspiration and eerie similarity from the R&B singer Etta James. Despite her last name being shown on the album (and on her credit card during her failed attempt to leave New York City), she is still credited only as "Gretta".