Acclaimed evangelical scholar Richard Bauckham in his book on the gospels Jesus and the Eyewitnesses: The Gospels as Eyewitness Testimony (2006, Wm. B. Eerdmans Publishing Co) argues that the fourth gospel stems from an eyewitness to the ministry of Jesus, namely, the disciple John. At the same time, however, Bauckham also acknowledges that,
“All scholars, whatever their views of the redactional work of the Synoptic Evangelists and of the historical reliability of the Gospel of John, agree that the latter presents a much more thoroughly and extensively interpreted version of the story of Jesus.” p. 410. (Emphasis added)
Even if we are to accept the fourth gospel as a product of an eyewitness (and it should be noted that most experts disagree with Bauckham), it does not mean that we can simply read off from its surface the words attributed to Jesus as if Jesus literally uttered them during his ministry.
As John Drane says,
While all four gospels contain factual fictive elements, the fourth gospel appears to have a greater preponderance of the latter.’
John Drane, Introducing the New Testament, Lion Publishing Plc. Revised Edition. 1999 pp. 210-211.