When we think about the Gospels we often think about written documents. Indeed, much of the study related to the Gospels has been on how they were written. But a growing number of New Testament Scholars over the last two decades have taken seriously the idea that before there was a written gospel, the story of Jesus was circulated orally.
James D.G. Dunn is one who has been working in this area beginning with his Jesus Remembered. (Eerdmans, 2003) His latest work is the Oral Gospel Tradition (Eerdmans, 2013). Below is an interview in which Dunn talks about how oral tradition impacted the creation of the Gospels.