Simile in Macbeth
The sergeant tells Duncan that Macbeth and Banquo attacked with no fear as they redoubled their efforts against the enemy. His simile compares the ferocity of their attack to cannons that have been loaded with extra explosive charges, and he states that he is not sure what motivated them to fight so hard.
Shakespeare uses a simile to explain the battle between the king's forces and the invading Norwegians and their Scottish-rebel allies. The image of two tired swimmers who hold on to each other to keep from drowning reveals how the soldiers of the two armies are exhausted and neither side seems capable of winning. The soldier wants King Duncan to know that victory looked uncertain until Macbeth exerted his leadership.
This simile tells the audience how tired Banquo is: he feels the call of sleep ("a heavy summons") weighing on him ("like lead") and making him tired. However, he ignores the call, for reasons we shortly learn in his conversation with Macbeth.
Lady Macduff's son jokingly answers her question with a simile which his mother then continues this idea of her son as a bird with an extended metaphor. His response of "With what I get, I mean; and so do they" means that he will get by however he can. Their witty repartee reveals a fond and loving relationship and makes the end of the scene all the more tragic.
Notice how Macduff and Malcolm both refer to Macbeth as black, evil, and a devil. Shakespeare likely used these particular words for the express purpose of giving the rebellion against Macbeth a moral and religious component. Macbeth is not just a bad king; he is an agent of the devil. This means that Malcolm and Macduff have God and righteousness on their side.