Both actors actually had music backgrounds. Teller started playing the piano when he was six. When he was older, he took up guitar and drums, and he continued playing the drums when he was attending NYU. Since Teller was already well-versed with the drums, much of his time was spent practicing the music.
According to Simmons, he was initially told by Chazelle that there would be a technical adviser on set that would help Simmons with any of the musical aspects of his character. The production was even considering used a body double to handle some of the conducting by Simmons’ character. To Chazelle’s surprise, Simmons told him that he had an undergraduate degree in music from the University of Montana where he studied voice, composition and conducting. Still, much of his background was in classical music, so to prepare for the role, Simmons spent an enormous amount of time researching jazz, not just listening to relevant recordings but also reading through the scores and growing the same familiarity with them that his character would have. Once they started filming, almost all of the playing was done live on-set.
It’s worth noting that Simmons doesn’t share his character’s views on teaching. At a press conference promoting the film, he stated “I completely agree with feeling the need for or the benefits of being pushed and of being directed on a project and collaborating. But, that kind of manipulation and abuse has no place in life.”