Question Restatement: "In Mark 9, why is the crowd, (who was with the disciples as they were trying to cast out a demon), "amazed" to see Jesus--since he had not done anything yet?
Amaze, from Merriam-Webster - - obsolete : bewilder, perplex.
Mark 9:14 - When they came back to the disciples, they saw a large crowd around them, and some scribes arguing with them. 15 Immediately, when the entire crowd saw Him, (they were amazed [alarmed/distressed]) and began running up to greet Him. 16 And He asked them, “What are you discussing with them?” 17 And one of the crowd answered Him, “Teacher, I brought You *my son, possessed with a spirit which makes him mute; 18 and whenever it seizes him, it slams him to the ground and he foams at the mouth, and grinds his teeth and stiffens out. I told Your disciples to cast it out, and they could not do it.”
Answer: The context indicates that they were not "amazed", in a "Joyful Sense", to see Jesus--but rather, they were already "in shock/distressed" because of the disciples inability to cast out the demon, and so they "ran" [translation?] to Jesus in desperation.
Greek Analysis, ἐξεθαμβήθησαν Indicates Distress
The Greek word for "Amaze", here, does not connote a positive sense, "of joy". But, rather "ἐξεθαμβήθησαν" indicates "alarmed/distress/troubled", as the disciples could not heal the boy, and so because of their distress, ran to Jesus in desperation. This is the same word used in Mark 16:5, and in Mark 14:32, (notably without a sense of joy).
NOTE: Thayer's analysis, here, is a little limited, but the complete contexts clarify the meaning.
Mark 16:5, NASB - 5 Entering the tomb, they saw a young man sitting at the right, wearing a white robe; and they were amazed. 6 And he *said to them, “Do not be amazed [in Matt. 28:5, "do not be afraid", from φοβεῖσθε]; you are looking for Jesus the Nazarene, who has been crucified. He has risen; He is not here; behold, here is the place where they laid Him.
Mark 14:32, NASB - They *came to a place named Gethsemane; and He *said to His disciples, “Sit here until I have prayed.” 33 And He *took with Him Peter and James and John, and began to be very distressed and troubled. 34 And He *said to them, “My soul is deeply grieved to the point of death; remain here and keep watch.” 35 And He went a little beyond them, and fell to the ground and began to pray that if it were possible, the hour might pass Him by.