THE SYNOPTIC PROBLEM
No history has ever been written that records dialogue. It is impossible to comprehensively do so. For none knows the importance of an event until well afterwards. The Gospels comprehensively record the dialogue of events.
Not all witnesses see and hear the same things and so they record different aspects of an event.
Large parts of the Gospels are oratory. A good orator uses repetition. Hence verbal disagreement at least must be expected,
as an idea is repeated in different forms.
Sources for Gospel material may be many. Individual Gospel writers, bystanders, those healed, Temple records, civil records or the obvious one, Jesus himself as he spent 40 days with the disciples before his ascension, in which time they could easily have written down all that he wanted them to say.
The Holy Spirit is said to be the source for it is said.
the Comforter, the Holy Ghost, … he.. .shall… bring all things to your remembrance, whatsoever I have said unto you. Jn. 14:26
flesh and blood hath not revealed it unto thee, but my Father which is in heaven. Mt. 16:17
It must be noted that some events had no notable witnesses other than Christ. There were no disciples other than John at the religious trials of Christ. Yet the other writers record them and that different to John and each other. Therefore, there was no literary dependence, copying, there. And other writers were, at least in this instance, not the source of any one other.
There were events recorded where no followers of Christ were present e.g. the temptations.
There is agreement in basic order of each Gospel. This largely rules out an oral source, which would essentially be without chronological indicators.
A brief overview of the generally accepted views on the Synoptic Problem are given in these links. If you have little understanding of the problem it would pay to look at these sites first, which generally give an accepted view of the situation, but which is at variance to what is contained on my website. bible.org/article/synoptic-problem and www.blueletterbible.org/faq/q.cfm.
PAPIAS was the first to say that there was an oral collection of
sayings of Jesus before the Gospels were written. This implies that
the Gospels could not be chronogically written, nor accurately
transmitted. This assumed collection of sayings on Papias'
testimony, whether in written or oral form is the basis for the
assumption that there is a
Q source. (German quelle -source)