It seems to me that the most interesting aspect of conscience is in the character of Claudius. All his heavy drinking is probably due to his guilty conscience. He is drowning his memory and his fear of retribution in drink. The scene in which he is trying to pray contains a full expression of his guilty conscience. When Hamlet stages the play within a play, Claudius is horrified to see his own crime enacted in front of him. His guilty conscience causes him to rise and flee the room. Claudius voices a very interesting aside in Act 3, Scene 1, after Polonius says:
We are oft to blame in this—
'tis too much proved—that with devotion's visage
And pious action we do sugar o'er
The Devil himself.
Claudius says to himself in an aside:
O, 'tis too true!
How smart a lash that speech doth give my conscience!
The harlot's cheek, beautied with plastering art,
Is not more ugly to the thing that helps it
Than is my deed to my most painted word.
O heavy burden!
In fact, Claudius is an excellent example of how a guilty man would act in this cunning usurper's situation. Hamlet is the only character in the whole play who knows Claudius is guilty and acting unconcerned to cover up his true feelings of guilt, remorse, suspicion, fear of exposure, and fear of divine retribution. This makes him a multidimensional character. It must be a difficult role for an actor. A literary character with a comparable problem is Rodion Raskolnikov in Fyodor Dostoyevsky's classic novel Crime and Punishment. Georges Simenon frequently deals with the theme of concealed guilt, not only in his Inspector Maigret novels but in his psychological novels such as The Murderer.
When Hamlet first talks to the Ghost, he intends to give all of his attention to avenging his father by killing Claudius. However, the play is extended as/because Hamlet delays this multiple times. Perhaps the main reason Hamlet delays is that the moralizes and examines the philosophical implications of his revenge too much. Given that conscience is the notion of ethics (what is right and wrong), Hamlet is always engaging his conscience almost as if he is in a constant debate with it (or with himself). One of the most overt events in which Hamlet consults his conscience is after he discusses the play with the players in Act Two, Scene Two. Not only is his own conscience something he must consider; he must also consider Claudius conscience. Hamlet intends to avenge his father but he also wants to expose Claudius' guilt to himself and to others:
The play's the thing wherein I'll catch the conscience of the king. (II.ii.599-600)
Hamlet has an opportunity to kill Claudius in Act Three, Scene Three, but again he delays. He considers his own conscience as usual (is this the right time to kill). But he also considers Claudius' conscience. If he kills Claudius while he (Claudius) is praying, it will not be a satisfying revenge because Claudius would go to heaven, being killed in the righteous act of praying:
Now might I do it pat, now he is praying;
And now I'll do't. And so he goes to heaven,
And so I am revenged. That would be scann'd.
A villain kills my father, and for that,
I, his sole son, do this same villain
send to heaven. (III.iii.75-80)
Once again, Hamlet must consider his own conscience as well as Claudius'. This double consideration of conscience doesn't extend to all other characters, but the theme is still significant. Hamlet does not consider Ophelia's conscience/state of mind and therefore alienates her, helping lead to her suicide. Whether Hamlet focuses too much on his conscience, too much on the conscience of others (Claudius), or not enough on others (Ophelia), his obsession with conscience is what propels and delays the play. In other words, Hamlet's conscience paradoxically calls for revenge but makes him delay that revenge.