One of the most beloved arias of all time, "Erbarme dich" ("Have mercy on me") comes straight from the contrite heart of Peter, the disciple of Jesus, on Good Friday. After denying Christ three times, he realized his sin, and "went out and wept bitterly". The violin solo represents the anguish of Peter's soul at this moment. Bach scores the emotion here, just like a movie composer would. This is musical storytelling at its finest.
Thank you to listener Molly for the suggestion!
Hear "Erbarme dich" performed by the Netherlands Bach Society (Tim Mead, alto; Shunske Sato, violin solo; Jos van Veldhoven, conductor): https://youtu.be/ZwVW1ttVhuQ?t=5179
Other music from the St. Matthew Passion: "O Mensch, bewein" (played in the background near the end of the episode): https://youtu.be/ZwVW1ttVhuQ?t=3818