"He left the actors during the performance."
Translation:Han forlod skuespillerne under forestillingen.
Grammatically it is correct, so you might say it but it does not sound quite right to me. "i løbet af" expresses something that happens over time. So "Han forlod skuespillerne i løbet af forestillingen" somehow implies that it took him some time to leave. That said, you would absolutely be understood. Some might not even agree with me.