"Theissen argues for the most part (and sufficiently) persuasively that the pre-Markan passion narrative as a whole was in use by A. D. 40 in Jerusalem and Judea. Thus, for example, Mark preserves names (such as the sons that identify the second Mary and Simon) that serve no recognizable function in his own narrative--but that may well have been recognizable to those who transmitted his early Jerusalem source (Mk 15:21, 40). Place names like Nazareth, Magdala, and Arimathea would mean nothing to audiences outside Palestine. (Regarding Mary Magdalene, who would invent an origin in Magdala? A Magdala appears often enough in later rabbinic sources, but it was not known by this name outside Palestine.)"
But that's not all."Although one normally identifies local persons through their father's name, most persons in the passion narrative (which identifies more people 'than elsewhere in the synoptic tradition') are identified by their place of origin instead. This practice makes the most sense in the church's first generation in Jerusalem, when (and where) its leading figures were people from elsewhere. Mark presumes his audience's prior knowledge of Pilate and (more significantly) Barabbas and other insurrectionists, despite Pilate's confrontation with a wide array of revolutionaries. Finally, some central characters in the account remain anonymous, probably to protect living persons who could face criminal charges in Jerusalem, fitting other ancient examples of protective anonymity. Taken together, these arguments seem persuasive."
"Another line of evidence also supports the substantial reliability of the picture of Jesus’ execution found in the passion narrative: it fits what we know of the period in question. Thus Craig Evans compares Josephus’ account of Joshua ben Hananiah, who similarly entered the Temple area during a festival. Like Jesus, he spoke of doom for Jerusalem, the sanctuary and the people, even referring (again like Jesus) to the context of Jeremiah’s prophecy of judgment against the Temple. The Jewish leaders arrested and beat Joshua and handed him over to the Roman governor, who interrogated him. He refused to answer the governor, was scourged, and—in this case unlike Jesus (though cf. Mk 15:9)—released. The different outcome is not difficult to account for: unlike Joshua, Jesus of Nazareth was not viewed as insane and already had a band of followers, plus a growing reputation that could support messianic claims. Joshua ben Hananiah could be simply punished in an attempt to deter his continued antisocial behavior; Jesus of Nazareth had to be executed." (Keener is referencing an essay by Craig Evans called "What Did Jesus Do?" in Jesus Under Fire edited by Michael Wilkins and J. P. Moreland. Oh, you may be wondering, like I did, whether there could be a literary relationship between Josephus and the Markan passion narrative since there are so many parallels between them. Evans provides a couple of reasons why he thinks there wasn't one. See his essay p.114n30.)I believe, with Keener, this presents a fairly strong case for the reliability of the pre-Markan passion narrative.