Jesus is publicly asked “Are you the Messiah (Christ)?” and, in an unprecedented manner (for Mark’s narrative), he openly proclaims “I am”. The secret is out. Then, when he is brought for another hearing before Pilate, the Roman governor, the issue of identity is at the fore: “Pilate asked him, ‘Are you the King of the Jews?’ He answered him, ‘You say so’” (15:2 [NRSV]) — which amounts to a “yes” here.But does the latter amount to a yes? Given that Jesus does say such an ambiguous yes in front of the high priest in 14.62, why not here in front of Pilate? What does σὺ λέγεις mean?
Perhaps one clue lies in the different questions asked by the high priest and by Pilate. The high priest's question is "Are you the Christ, the son of the blessed?" Now, we know that these are titles Mark approves of -- 1.1 announces the beginning of the gospel of Jesus Christ; the baptism and the transfiguration have God announcing that Jesus is his son; and Peter (Chapter 8) and the nameless woman (Chapter 14) recognise that Jesus is the Christ. The only thing that needs adding is some qualification -- he is anointed to suffer (8.31), or for burial (14.3-10), and now the confession that he is the Christ leads directly to his death.
But Pilate's question is different, "Are you the king of the Jews?" (15.2). Nowhere does Jesus own the title "king" in the Gospel, though it is the one that everyone imposes on Jesus throughout the Passion Narrative, king of the Jews, crown of thorns and so on. This is one of those places, I think, where John is a fine exegete of Mark and he teases out the meaning of the terse, ambiguous "You are saying so" in this way:
Pilate then went back inside the palace, summoned Jesus and asked him, "Are you the king of the Jews?" "Is that your own idea," Jesus asked, "or did others talk to you about me?" "Am I a Jew?" Pilate replied. "It was your people and your chief priests who handed you over to me. What is it you have done?" Jesus said, "My kingdom is not of this world. If it were, my servants would fight to prevent my arrest by the Jews. But now my kingdom is from another place." (John 18.33-36)