All the subway scenes are filmed in Bucharest where the old trains run filled with graffiti except the windows.
In addition to having to learn the violin, Galitzine had to learn how to Tango. And, he had to lead a professional dancer in the Latin dance. About this he said: "We had like about a week or something before that of learning all the moves. And it's very difficult because in a Tango, the man's supposed to lead, and, you know, if you're not a dancer and they kinda tell you to lead this very accomplished dancer, it's kind of scary," Galitzine admitted, adding that Keenan "is an exquisite dancer in kind of pretty much all forms," including, "a pretty mean robot."
Nicholas Galitzine (Johnnie) was unfamiliar with the violin before landing the part. Prior to his casting, he had only played guitar. Galitzine commented: "They gave me some time before and during the filming where I had very extensive crash course training. I had an amazing tutor in London, and then throughout the filming, sort of day and night whenever I wasn't filming I was kind of practicing to make it authentic."
Keenan Kampa, a trained ballerina, was the first choice for the role of Ruby, but when Michael and Janeen Damian contact her and ask her if she had any interest in acting, she was recovering in a hospital from a hip surgery. She had to do three sessions of rehab a day to get back into ballet shape.
TwoSet Violin, a duo of professional violinists on YouTube, made a video critiquing the violin-playing scenes in the movie called "The WORST Violin Movie Acting We've Ever Seen." The part they found most egregious is the dueling scene, where two characters strike at each other's bows with their own (because that would severely damage the bows--and a professional soloist bow costs upwards of $50,000).