What factors tend to increase the reliability of recollective memory, and how do these relate to the historical reliability of the Gospels?

Richard Bauckham identifies the following:

1. Unique or unusual events. The gospel stories clearly fit this category, as many of the events, such as healings and exorcisms, are unmistakably unusual.
2. Salient or consequential events. Gospel stories often involve landmark or life changing events that would create vivid memories in people who witnessed them.
3. An event for which a person is emotionally involved. The gospel writers were not dispassionate observers, but were personally invested and emotionally involved in the events themselves. They were deeply affected by the events.
4. Vivid imagery. The gospel stories have little vivid imagery. Mark tends to have more than the other gospels, but it is difficult to know if this imagery is the result of Mark being close to the events or being a good storyteller.
5. Irrelevant details. There are some irrelevant details in the Gospels, but most details have been preserved because of their significance and memorability. However, as Bauckham notes, the lack of irrelevant details is not evidence against eyewitness provenance.
6. Point of view. People remembering events and stories often switch point of view in how they tell the story. And this occurs regularly in the Gospels. The gospel of Mark, for instance, does this to show that Mark is preserving the eyewitness testimony of Peter.
7. Dating. People typically remember details about events, such as location, actions, time of day, emotions, and persons involved, but dates are not common. The Gospels fit this characteristic of memory, in which the recorded events only include indications of dating for specific reasons.
8. Gist and details. The overall gist of a memory is likely to be accurate, even if details vary. We see this pattern in the Gospels (e.g., Matt 26:58, 69–75; Mark 14:54; Luke 22:54–62; John 18:15–18, 25–27).
9. Frequent rehearsal. Stories that were told frequently were more likely to become standardized in a certain form and better remembered. As Bauckham notes, we can be sure the apostles told the stories of Jesus frequently after the events.

Bauckham, Jesus and the Eyewitnesses (2nd ed.), pp330-335 (as summarised in McDowell & McDowell, Evidence That Demands a Verdict 2017 ed., ch. 3.)