Since Mark was able to retrieve the actual text without difficulty, this would argue that shorthand writers must have been present to record the lectures. This need not surprise us since the shorthand recording of public statements of senators and prominent officials was commonplace in first-century Rome; indeed, it had been a recognized practice ever since the time of Cicero a hundred years earlier. At any rate, Clement of Alexandria makes it clear that Mark was able to come by a verbatim report of what Peter had said, since Mark was able to supply the text on request. Furthermore, the Gospel of Mark is in no way the smooth product of a skilled author seated at his desk, but has all the vividness and peculiar turns of speech that one finds in actual transcripts of live speeches.
from pages 66-67, Why Four Gospels?