Early in the play, Malcolm doesn't really say or do much. He takes direction from his father and seems to try to be as loyal and obedient as possible to Duncan, his king and father. Even when his father names him Prince of Cumberland, the heir to the Scottish throne, Malcolm says nothing, so we might assume that he is speechless. In general, he seems young, naive, and inexperienced.
By the time Macduff comes to speak with him in the English court, however, Malcolm has clearly become much more worldly. Malcolm wisely tests Macduff's loyalty to Scotland before speaking truthfully with him about the army with which the king of England has supplied him. He says, "Devilish Macbeth / By many of these trains hath sought to win me / Into his power, and modest wisdom plucks me / From overcredulous haste" (4.3.136-139). He has learned from his experience to be wary. His suspicion and intelligence speak highly of his fitness to rule Scotland, especially compared to Macbeth's willingness to believe anything the Weird Sisters say, for example, and the way he privileges power over wisdom. Malcolm longs to serve his country, not rule over and exploit it. He has developed into a mature, virtuous, and thoughtful young man.