Albert Barnes Commentary Jeremiah 36:16

"Now it came to pass, when they had heard all the words, they turned in fear one toward another, and said unto Baruch, We will surely tell the king of all these words." — Jeremiah 36:16 (ASV)

They were afraid both one and other - literally, “they trembled each to his neighbor,” that is, they showed their alarm by their looks and gestures one to another. They felt that what he had so consistently prophesied for a period of 23 years would in all probability be fulfilled.

We will surely tell - Rather, We must tell the king. It was their official duty.