Guy Montag met Faber for the first time once when Montag was walking through a park. He saw Faber look guilty and try to hide something so he went over to him. But instead of arresting him or anything like that, he just talked to him. They talked for a long time and then Faber finally gave Montag his name and address in case Montag wanted to turn him in to the authorities later on.
In his previous life (before such things became illegal) Faber had been a professor. Specifically, he had been an English professor.
Guy meets Faber in a park a year before the events in the novel take place. This flashback foreshadows Montag's eventual desire to leave the social constrictions imposed on all citizens. Montag initiates the conversation by starting with a banal discussion of the weather. Eventually, while sitting beneath the trees in the park, Faber "lost his fear of Montag" and "talked in a cadenced voice, looking at the sky and the trees and the green park, and when an hour had passed he said something to Montag and Montag sensed it was a rhymeless poem."
This conversation, in which Montag learns that Faber worked as an English professor at a liberal arts college, ends with Faber handing Montag his contact information "in case you decide to be angry with me."
When Montag gains curiosity about the book he's stolen, Faber is the only person he can think to approach. The fact that Montag had filed Faber's contact information under "FUTURE INVESTIGATIONS (?)" and had never turned him in indicates that Montag's decision to become a lawbreaker wasn't a random event. While Clarisse helped prod Montag along his course, Montag himself had seeked out information a year before.