The characters learned how to play the instruments.

In some of the episodes, the lead characters had to play real instruments. Mayik Bialik actually had to learn how to play the harp for her character Amy and Jim Parsons had to learn the ancient theremin for his character Sheldon. Johnny Galecki, however, already knew how to play the piano and is even a cellist in real life. No wonder Kaley Cuoco was fascinated by him!