Why does Hamlet think he has the "motive and cue for passion"?
In this soliloquy, "O, what a rogue and peasant slave am I," Hamlet reflects on the actor he has just seen playing a part. Hamlet can't get over how well this man, who plays the part of a woman, Hecuba, is able to pretend to emotions he doesn't really feel. He cries, shows distress, has a "broken voice," all to play the part of a woman from thousands of years ago who lost her husband in the Trojan war. What can Hecuba possibly mean to this actor, Hamlet wonders. Why should he "weep for her?"
Then Hamlet compares this actor's faked grief to his own real grief, asking what the actor would do if he had "the motive and the cue for passion" that Hamlet had. This statement means that Hamlet has a "motive"--a reason--and a "cue"--a prompting--to be upset that is quite real. "Cue" is a pun on an actor's cue--what someone will whisper to an actor off stage so he knows it is his time to act. Hamlet has both the death of his father and the cue from the ghost that his father was murdered to drive his emotions into a frenzy, and yet he is paralyzed, can't act. He is very upset with himself over this state of affairs. "Why, what an ass am I!" he cries.