Albert Barnes Commentary John 1:36

"and he looked upon Jesus as he walked, and saith, Behold, the Lamb of God!" — John 1:36 (ASV)

Looking upon Jesus ... - Fixing his eyes intently upon him. Singling him out and regarding him with special attention. Contemplating him as the long-expected Messiah and Deliverer of the world. In this way all ministers should fix the eye upon the Son of God, and direct all others to him.

As he walked - While Jesus was walking.