Assistant engineers, sometimes called second engineers or 'tape ops', traditionally learnt the ropes in the studio by starting and stopping the tape machine whilst observing the first engineer at the recording console. They listened more than they asked questions.
Although operating a tape machine sounds simple enough, tape ops were often put under pressure. Peter Mew, a mastering engineer I interviewed at Abbey Road Studios in 2013 before his retirement, described how classical producers would ask him to 'go back four bars' during playback sessions even though bar numbers weren't marked on the tape itself. The task was one of translation, working out how far to rewind according to the tape speed and tempo of the performance.
It wasn't just classical playbacks where these sorts of requests were made. There is alarming footage of the studio sessions for John Lennon's song "Oh Yoko!", held in the summer of 1971, which shows Lennon berating engineer Philip McDonald for repeatedly dropping him into the wrong part of the song. Phil Spector remains quiet as he sings backing vocals alongside Lennon but the tension is palpable. Weathering the tantrums of artists is just one of the occupational hazards of the recording studio.