In the 1950s and ’60s, especially, session musicians could make or break a hit. And session musicians were in high demand, as producers like Phil Spector became obsessed with production techniques such as the Wall of Sound, forcing as many musicians as possible into a studio and having each of them contribute a small part to a larger, bombastic sound. As a result, session musicians became highly valued: Each had to play their role well, but they also had to find a way to click with every other session musician in the room. – Pacific Standard