When Jesus saw Nathanael approaching, he said of him, “Here truly is an Israelite in whom there is no deceit.” “How do you know me?” Nathanael asked. Jesus answered, “I saw you while you were still under the fig tree before Philip called you.” Then Nathanael declared, “Rabbi, you are the Son of God; you are the king of Israel.” Jesus said, “You believe because I told you I saw you under the fig tree. You will see greater things than that.” He then added, “Very truly I tell you, you will see ‘heaven open, and the angels of God ascending and descending on’ the Son of Man.”
The fig tree is a symbol of Israel. The story of angels ascending and descending refers to Jacob's (ie Israel's) ladder.
I suspect what was happening was that under the fig tree, Nathanael was discussing the story of Jacob's ladder. Jacob was notorious for his trickery. He had tricked his father and his brother by stealing the birthright and then having been tricked by his uncle Laban managed to trick him back. This would explain why Jesus called Nathanael 'an Israelite in whom there is no deceit'. And why Nathanael was so astonished and beguiled by Jesus.