Jumping to the second chapter of my Jesus in the Gospels book. This is on the special themes in Mark.
A Difference of Eyewitness Perspective?
Each of the four Gospels tells the story of Jesus in a distinctive way, although the first three gospels tell the story similarly enough to be called "synoptic" gospels, gospels similar enough that you could put them side by side and compare them. But from early on it was recognized that John went its own way. A Christian in the late 100s called it a "spiritual" gospel.  It is interesting to wonder how our picture of Jesus might be different if we didn't have John.
Sometimes people make sense of the distinct presentations of the Gospels with the picture of four people standing on four different corners of an intersection, describing what happened to cause an accident. In this common sense scenario, each person sees something a little different because they are looking on from a different angle. They are all describing the same thing, but their narratives are each going to be distinct.
There may be some truth to this scenario, but things probably are also a little more complicated.  It's true that, traditionally, Matthew and John were thought to be written by the disciples with those names. Mark's Gospel is traditionally linked to Peter, and it is often suggested that Luke did some serious eyewitness research, perhaps including Mary herself. I personally think that traditions like these have to be taken more seriously than many do, but I also acknowledge that such traditions regularly get a little mangled in process. 
As we will see throughout our journey, Matthew and Luke probably used Mark as their main source. The places where they differ from Mark are thus probably intentional and reflect special emphases they wanted to bring out more than differences in eyewitness perspective. Matthew and Luke are then thought to have relied on yet another written source as well. So while eyewitness testimony may very well underlie the foundations of the Gospels, most of the differences more come from the editing of the Gospel writers in order to bring out special themes.
Although we don't know Mark's sources for sure, it's likely that he also is telling the stories of Jesus in such a way as to bring out some special themes. The earliest tradition, about Mark, from the early part of the 100s, is that
"Mark having become the interpreter of Peter, wrote down accurately whatsoever he remembered. It was not, however, in exact order that he related the sayings or deeds of Christ. For he neither heard the Lord nor accompanied Him. But afterwards, as I said, he accompanied Peter, who accommodated his instructions to the necessities [of his hearers], but with no intention of giving a regular narrative of the Lord's sayings. Wherefore Mark made no mistake in thus writing some things as he remembered them. For of one thing he took especial care, not to omit anything he had heard, and not to put anything fictitious into the statements." 
The sense here is that the stories of Mark aren't necessarily in the order they happened. There is also the sense that Peter presented the stories in such a way that they would speak optimally to his audiences. Although we have to remember that Papias himself may have his own agenda in the way he presents this information, it is striking that this statement probably comes within 50 years of Mark's writing...
 Clement of Alexandria
 For a strong defense of the eyewitness basis for the Gospels, see Richard Bauckham, Jesus and the Eyewitnesses.
 Roberts-Donaldson translation currently, fragment VI.