What transforms an acquaintance into a friend?

Everyone has acquaintances -- we are all acquaintances here on line. This relationship, acquaintance, is a far cry from friend and friendship, conditions that imply trust, intimacy, comfort, sharing and so on. But what is it that tips the scale?

What is it that transformed for you one of your acquaintanceships?

Was it one major event that sealed the deal or a series of encounters that slowly built your friendship out of the rough material of acquaintanceship?