Jamieson, Fausset and Brown on Mark 9:20

20. And they brought him unto him: and when he saw him, straightway the spirit tare him--Just as the man with the legion of demons, "when he saw Jesus, ran and worshipped Him" (Mr 5:6), so this demon, when he saw Him, immediately "tare him." The feeling of terror and rage was the same in both cases.

and he fell on the ground, and wallowed foaming--Still Jesus does nothing, but keeps conversing with the father about the case--partly to have its desperate features told out by him who knew them best, in the hearing of the spectators; partly to let its virulence have time to show itself; and partly to deepen the exercise of the father's soul, to draw out his faith, and thus to prepare both him and the by-standers for what He was to do.