The conducting scene was shot on-site at the Ely Cathedral, initially with five different cameras; Bernstein is standing in the center of the orchestra so coverage from many angles makes sense. As Cooper recounts to Stone (and Variety), the rest of the movie used only two cameras and the turnaround time on scenes was rather quick, but this was a different animal.
The actors appearing as Bernstein’s orchestra were all professional musicians (Cooper says, “They’re the best at what they do. The concertmaster showed me a still of him at four years old with two wooden spoons like, ‘That’s when I started.'”).
So now, for the scene’s sake, Cooper had to conduct this A-Team orchestra in the craft they’d excelled in. The eyes of the camera were all on him, so there was no getting around the performance, and the resulting music had to sound good to not break the scene’s immersion.
Cooper was, as Stone put it, “in the finals” like an athlete before a big game, and feeling all the pressure that brings. He also admits that conducting is “the hardest thing.” That he was doing it in front of professionals, while playing a legendary conductor, only added to his embarrassment as he struggled to keep up. As Cooper tells it:
“[As a conductor], you’re keeping tempo a little bit ahead to make sure, and you’re also reminding everybody of these things that you’d worked on in rehearsal. Luckily they’re the greatest orchestra in the world. But still, I had to conduct them because the shot is seeing everybody. So I messed up the whole first day. I kept messing up. And the minute I was behind tempo, it’s over. Like, I’m lost.”