Hamlet is very concerned with how to go about avenging his father's death. He decides to take revenge by killing Claudius, but he constantly delays taking action because he is consumed with the best way to exact his revenge. In this way, the play is very much about Hamlet's thought processes (the "method to his madness") as he hems and haws about how to act. This general theme is often called Hamlet's delay or procrastination.
In Act III, Scene 3, we see this theme of thought and action in both Claudius and Hamlet. Claudius is alone attempting to pray, but his guilt is preventing him from doing so. "My stronger guilt defeats my strong intent." Hamlet approaches, unseen by Claudius, and thinks this is the time to kill. But Hamlet delays, thinking that if he kills Claudius while he is praying, he may go to heaven and with a clear conscience since he is asking forgiveness and praying to God:
But in our circumstance and course of thought,
tis heavy with him; and am I then revenged,
To take him in the purging of his soul,
When he is fit and seasoned for his passage? (III.iii.85-89)
Hamlet's soliloquies also illustrate this flux between thinking and acting. His famous "to be or not to be" speech demonstrates his anxiety of whether to end his life or to stay alive in grief and/or in order to take revenge on Claudius.
Interestingly, Hamlet's delay is what makes the play. In other words, had he killed Claudius in Act One, the play would be over. In a sense, Hamlet's delay and manipulation of other characters drags things out - it is as if he is taking part in writing the events, albeit with some outcomes he does not plan for.