1. The imprisonment of John.
While John himself has been introduced (in 1.2-6), nothing has been said which would imply that he was going to be imprisoned. Therefore, this notice seems to presume readers will already know about John's imprisonment, in much the same way that John seems to presume that his readers will know about it:
(ETA: John 1.15 is similar insofar as John the Baptist is claiming to have preached something which the gospel of John itself has not narrated; but the synoptic gospels do.)
Naturally, readers of John may well have known of this fact from previous gospels, but the notice in Mark 1.14-15 suggests that readers of Mark, as well, are expected to have known it. The notice itself is of the kind found frequently in the Hebrew scriptures whereby the narrative or oracle at hand is dated with reference to a well known event:
Likewise, for readers of Mark, the imprisonment of John seems to have been a known event.
2. Simon Peter.
Mark 1.16 seems to presume that readers will already know who Simon is. Unlike most characters in the gospel, Simon is given no introduction by nickname, patronymic, or any of the usual manners; and his brother, Andrew, is identified by his relationship to Simon. Refer to my post on named characters in Mark for more information: viewtopic.php?f=3&t=2551.
3. The son of man.
The gospel of Mark uses the title "son of man" in a way which seems to expect its readers already to know what it means. Mark 2.10 and 2.28 may be using the phrase "son of man" to mean "human," which is one of its main functions as a Semitic idiom. But in Mark 8.31 it means something more, and this "something more," as a title for Jesus, is never really explained, leaving modern scholars to write entire monographs on the topic.
4. The disciples of John.
Mark gives the reader no early indication that John might have disciples. His description of the prophet is as a loner in the desert: surrounded by crowds, to be sure, but not calling them to himself or instructing them as a mentor instructs pupils. So that John has disciples comes a bit abruptly:
To be fair, however, all groups in Mark seem to be introduced abruptly: "the scribes" (1.22), "the scribes of the Pharisees" (thus introducing the Pharisees themselves, as well, 2.16), and the Herodians (3.6). So the introduction of groups may not follow the pattern of introducing individual characters.
ETA: Stefan points out that the Sadducees are actually a group which Mark gives something of an introduction for. I missed that.
5. The betrayal by Judas.
In the list of disciples, long before Judas has betrayed his Lord, Mark already mentions that betrayal:
This is very similar to how John introduces Mary, even before she has anointed Jesus:
This anointing will not take place until a chapter later, in 12.1-11. It is as if John expects his readers to already know this story (from earlier gospels, at least), and he is merely pointing out that this is that Mary.
Similarly, it is as if Mark expects his readers to already know the story of the betrayal by Judas, and he is merely pointing out that this is that Judas.
Compare also how Genesis 36.9 mentions the Edomites, descendants of Esau, long before any point in the narrative where Edomites should exist yet. It is as if the author is pointing out that this is the Esau who was the ancestor of those people, the Edomites, whom the readers are expected to know. Or compare how Josephus points out in Antiquities 1.13.2 §226 that the mountain upon which Abraham had been going to sacrifice Isaac was the same mountain upon which the Temple was later built. Despite being yet seven books away from describing the building of the first Temple, Josephus takes the time to connect Abraham's action (within narrative time) to something with which his Roman readership would already be familiar (without narrative time): the Temple Mount. (Josephus does not seem to expect his readers to know the exact history of the Temple, but he does expect them to know what the Temple is.)
Pilate, like Simon Peter, is one of the characters in Mark who needs no introduction: viewtopic.php?f=3&t=2551. He comes in unannounced:
Luke 3.1 and Matthew 27.2, on the contrary, give Pilate a proper introduction into the narrative. But Mark is hardly the only Christian who thinks he requires none. Many other Christian statements, especially some of a somewhat credal nature, also speak of Pilate as a known entity:
In Against Heresies 2.32.4 Irenaeus writes of the church receiving gifts from God "in the name of Jesus Christ, who was crucified under Pontius Pilate." In Against Heresies 3.4.2 he writes of Jesus having been "born of the virgin," having united man to God, and having "suffered under Pontius Pilate." In Against Heresies 3.12.9 he writes that the revelation which Paul received was of the one "who suffered under Pontius Pilate." In Against Heresies 4.23.2 he writes of Philip persuading the Ethiopian to believe in Jesus, "who was crucified under Pontius Pilate." In Against Heresies 5.12.5 he writes that Paul preached "the gospel of Jesus Christ the son of God, who was crucified under Pontius Pilate."
Mark's first mention of Pilate is every bit as abrupt as the creeds' mentions of Pilate are, suggesting that his readers already knew under whose authority Jesus was crucified.
7. Alexander and Rufus.
The book of Ruth gives, with the birth of Obed, a miniature genealogy leading down to David:
Clearly, readers of this book are expected to already know who David is, and the author is merely telling them that Obed happens to be this famous David's grandfather. In a similar manner, readers of Mark are expected to know who two sons of one of the supporting characters is:
This kind of jumping out of the narrative to mention later people or events which depend in some way upon what is happening in the narrative is a fairly common storytelling device: viewtopic.php?f=3&t=3125&p=71433#p71433. In this case, Alexander and Rufus, while unknown to us, must have been known in some way to the first readers of this text.
8. The second Mary.
Mark 15.40 seems to presume that readers will know how to sort out the names of the women: Μαρία ἡ Μαγδαληνὴ καὶ Μαρία ἡ Ἰακώβου τοῦ μικροῦ καὶ Ἰωσῆτος μήτηρ καὶ Σαλώμη. The issue is that second Mary. The Greek wording is capable of being understood in six different ways, including three in which two separate women are in view:
- Mary (the wife) of James the Less and the mother of Joses.
- Mary (the daughter) of James the Less and the mother of Joses.
- Mary the mother of James the Less and of Joses.
- (A) Mary (the wife) of James the Less and (B) the mother of Joses.
- (A) Mary (the daughter) of James the Less and (B) the mother of Joses.
- (A) Mary the mother of James the Less and (B) the mother of Joses.
To summarize, I think that the author of the gospel of Mark was writing for readers who already knew at least certain parts of the story. One part of the story involves John the baptist, since readers are expected to know both that he was imprisoned and that he had disciples. Another part of the story involves the crucifixion of Jesus, since readers are expected to know who Pilate is, that Jesus was betrayed, who Alexander and Rufus are, and at least something about the women at the cross. There may be other presumed parts of the story that I have not sussed out yet. The title "son of man" may not be a story element at all, but rather an element of early Christian theology. And knowledge of Simon Peter may or may not include stories about him; he may simply have been known as a famous Christian apostle.
This analysis says nothing about whether what Mark's first readers knew came from historical facts, from legendary tales, or from previous gospel texts. Any or all of those options are left wide open, much in the same way that there are many different ways in which Josephus' readers might have come to learn about the Temple.