The Calling of the Disciples
Note that all these events in these few chapters occur around Bethany. Bethany is reputedly in the south east area (3 miles away) from the Mount of Olives, which is to the east of Jerusalem (check out Google Earth around the Jerusalem area with the "relief" option on to get an idea of the topography and geography). So John was baptizing in this area, Jesus had come to this area too from His home town of Nazereth, and the disciples he gathered here, Andrew, Simon Peter his brother, Nathanael, and Philip, and one other unnamed were staying in this area currently with John the Baptist. However, Andrew, Peter and Philip were from Bethsaida, a city by the north-east coast of Galilee. Two of them, Andrew and the unnamed disciple were followers of John the baptist, up to this point. Note also that these events occured in a matter of days. It appears that first, Jesus came to the area where John was baptizing, and He had lodgings there. On the day John baptize Jesus (from the gospels), John saw the vision of the dove descending on Him and identified Him as the Messiah (but we also know that John was aware of Jesus' identity for before He was baptized and John saw the vision, John already expressed humility in that he was hesitant to baptize Jesus because of his knowledge of His divinity). Note also that John adds translations of Hebrew words into Greek, perhaps being aware that his audience may not be fully Jewish.
Thus the two followers of John must have heard from John himself talk about the coming Messiah. It is therefore no surprise that when John identified Jesus as the Messiah, the two would follow Him. It also appears then, that Jesus did not just arrive on the scene out of nowhere, but that people had already a fair knowledge of His background, who He was as a person, being from Nazereth, and perhaps, He had already showed a little of His thinking in His conversations. For John was "looking at Jesus as He walked". Consider this for yourself, what are you looking at in life?
Perhaps, this event occured over a few hours, where as the two followers were with John, John talked about Jesus, and they considered what John was saying, and then decided to follow Jesus themselves. Regardless, the followers were rewarded by Jesus turning and asking "What do you seek?" This is perhaps a very difficult question for some. What do you seek in life? Some have no aim, or seek nothing. Some seek something that is material, some even seek after something that is spiritual. But perhaps, as long as that which we seek is not God, they all fall short of the ultimate desire. How interesting then, that the disciples' reply was to first acknowledge Him as teacher, and then apparently stating that they seeked to know where He stayed. But even that is enough. To know where the Divine stays. And they saw and stayed with Him. We can only guess the wonderful conversations they must have had during this time in the evening, for it was the tenth hour from sunrise.
The first response from this interaction for Andrew, was to find Simon, and to inform him that he had found the Messiah. Note again, everyone at that time was probably teetering on their toes looking for the Messiah, so this claim from his brother must have caught Simon's attention in some way. To his credit, Simon followed and was immediately declared prophetically by Jesus to be Cephas, or Peter, the rock.
Following this, Jesus wanted to return to Galilee, nearer His hometown. He had apparently come down south for certain reasons, perhaps the baptism, but had completed His purpose here for the moment. We are not told when the 40 days of temptation fit in, but presumably it was after the baptism and before He went back to Galilee. But prior to returning, He found Philip, who was from the same city as Andrew and Peter. So it appears these disciples were not random people, but people who had interacted with Jesus on various other occasions, and were chosen probably partly based on these interactions.
Philip's call to Nathanael again centers around Jesus being the Messiah. And Nathanael's response reveals the attitudes present towards Nazereth, and the inconsistency of a Messiah coming from there, since the Messiah is supposedly good.
The interaction between Jesus and Nathanael is somewhat a mystery, because it seems to be specific to Nathanael's experience. Nevertheless, John the author placed this section here, perhaps to emphasis how the disciples were convinced of Jesus' identity, and how Jesus revealed Himself to them. In Nathanael's case, it was through Jesus revealing His divine knowledge about him in a prophetic manner. Did Nathanael see something under the fig tree? Was he asking about something then? We are not told. But we are told that Jesus promised that he would see even greater things that this simple revelation of the Messiah through a prophetic word. He would see the Messiah Himself in His reconciliatory role (the angels of God ascending and descending are reminiscent of Jacob's vision of the ladder) and in glory.