What effect does shakespeare create by having Macbeth and Banquo enter just as they are discussing the execution of the former Thane of Cawdor?This...

Shakespeare tells us a lot, subtley, with the timing of his entrances and exits.  This particular instance is quite interesting because Duncan has just finished discussing the fact that he used to have "an absolute trust" in the former thane of Cawdor and in walks Macbeth, the new thane of Cawdor, whom Duncan immediately puts all his trust in without having learned his lesson from his prior mistake.  And, thanks to dramatic irony, we all know Duncan should not trust Macbeth!