What is the impression of Macbeth and what do we learn about Duncan's character in Act 1, Scene 2?
In Act 1, Scene 2, the Captain who reports on the battle to Duncan calls Macbeth "brave" and "Valor's minion" (1.2.18, 21). Despite the fact that the rebellion had fresh soldiers to replace those who were injured or dead and that Macbeth and Banquo had been fighting for so long, Macbeth remained steadfast and courageous, fighting until he slew the enemy. In doing so, Macbeth "unseamed him from the nave to th' chops, / And fixed his head upon [the] battlements" (1.2.24-25). He stuck his sword into Macdonwald's belly and jerked his sword upward, cutting him open from stomach to chin, and then he decapitated him, putting his head on a spike (as a warning to others). Thus, we learn that Macbeth is incredibly brave, terribly determined, and ruthless when necessary.
About Duncan, we learn that he is a compassionate king and a fair ruler. He commends the Captain for his service and bravery, saying that his words and wounds "smack of honor both," and he orders that the Captain be cared for immediately (1.2.48). Next, Duncan gives orders to execute the traitorous Thane of Cawdor and to give that title to Macbeth. Duncan rewards those who are loyal to him, just as he punishes those who are not. In contrast, when Macbeth becomes king, he keeps everything he can for himself, never rewarding those who cleave to him but all too willing to punish those he believes work against him. His tyranny is made starker by the contrast to Duncan's much fairer rule.
The scene, Act 1 Scene 2, is where we first learn about how brave and fierce Macbeth is. We first learn of him as a loyal subject, tireless in battle, courageous, and vicious (he does, after all, "unseam" a man from the navel to the "chaps"). We are told that even though the battle seemed to be lost, Macbeth never surrendered, but instead, fought harder than ever to get to the traitor and kill him. Here is the first reference to Macbeth not paying any mind to fortune or fate--it says he was "disdaining fortune". What this means is that Macbeth is already taking fate into his own hands. He is not the type of person to just give up and let life unfold whatever way it wants to.
In this scene, we learn that Duncan is a generous king who repays loyal service with honours. He is also a king who is not afraid to order a traitor put to death, so he is a capable ruler. He gives Macbeth the Cawdor's title because Macbeth has served him well and made money for his coffers in the process. Later on, we hear Duncan say something even more revealing about his character. Speaking of the first Cawdor, he says "there's no art to find the mind's construction in the face." This suggests he takes people as they appear to be on the surface. He does not know how to tell if someone is actually his ally or not. This proves to be part of the reason for his downfall.