John Proctor, The Crucible's protagonist, has some major issues. We can see why. Back in the day, he had everything your average Puritan man could want: a goodly farm to ceaselessly toil upon, three goodly sons to discipline, and a goodly wife with whom to make a home. Proctor was a stand-up guy who spoke his mind. Around town, his name was synonymous with honor and integrity. He took pleasure in exposing hypocrisy and was respected for it. Most importantly, John Proctor respected himself.
Enter: Abigail, the play's antagonist. This saucy, young housekeeper traipsed in, and, before John knew it, his goodly life was irrevocably corrupted. John made the mistake of committing adultery with her. To make things worse, it was also lechery, as Proctor was in his forties and Abigail was just seventeen. All it took was one shameful encounter to destroy John's most prized possession: his self-respect.
When we first meet John Proctor halfway through Act One, we discover a man who has become the thing he hates most in the world: a hypocrite. He is caged by guilt. The emotional weight of the play rests on Proctor's quest to regain his lost self-image, his lost goodness. Indeed, it is his journey from guilt to redemption, which forms the central spine of The Crucible. John Proctor is a classic Arthur Miller hero – a man who struggles with the incompatibility of his actions with his self-image. (Willy Loman of Death of a Salesman, Eddie Carbone of A View From the Bridge, and Joe Keller of All My Sons, all have similar issues.)
Adultery? Lechery? John, what got into you? Well, apparently John's wife Elizabeth was a little frigid (which she even admits), and when tempted by the fiery, young Abigail, John just couldn't resist. Elizabeth was also sick while Abigail was working for the Proctors, so she probably wasn't giving her husband much attention. More than likely, though, the cause of John's transgression is much deeper than base physical reasons.
It's also quite possible that John Proctor was attracted to Abigail's subversive personality. Miller seems to hint at this in the first scene in which we see them together in Act One. Abigail tells John that all the hullabaloo about witches isn't true. She and the other girls were just in the woods having a dance party with Tituba. Miller writes: "PROCTOR, his smile widening: Ah, you're wicked yet aren't y'! […] You'll be clapped in the stocks before you're twenty" (I.178). The key clue here is the stage direction. It seems to indicate that Proctor is amused and even charmed by Abigail's naughty antics. This would be in keeping with his personality. We see him challenging authority, from Parris to Danforth, throughout the play.
John Proctor is a passive protagonist; for the first two acts, he does little to affect the main action of the play. (Read more on this in "Character Roles.") By the time Act Three rolls around, however, he's all fired up. Spurred by his wife's arrest, he marches off to stop the spiraling insanity of the witch trials and to hopefully regain his own integrity in the process.
Proctor goes to court armed with three main weapons. There's Abigail's admission to him that there was no witchcraft. Also, he has Mary Warren's testimony that she and the other girls have been faking. Last, but not least, he's prepared to admit that he and Abigail had an affair. This would stain her now saintly reputation and discredit her in the eyes of the court. Between the wily machinations of Abigail and the bull-headedness of the court, all of these tactics fail. John only ends up publicly staining his good name and getting himself condemned for witchcraft.
Even though John doesn't achieve his goals of freeing Elizabeth or stopping the overall madness, he does take two significant steps toward regaining self-respect in Act Three. One: he doesn't stop fighting the false accusations even after he finds out that Elizabeth is pregnant and therefore safe for a while. He feels a greater duty to his community and proceeds anyway. Two: by openly admitting his adulterous lechery, he is no longer a hypocrite. He has publicly embraced his sin.
In Act Four, Proctor conquers the final hurtle on his path to redemption. This is no easy task; he stumbles a bit along the way. In order to save his life, he is tempted into admitting that he is indeed in league with the Devil. He justifies this lie to himself by saying that he's a bad person anyway. What's the difference? At least this way, he'll be alive. Of course, by doing so he's telling a terrible lie and is also blackening the names of all the other prisoners who've refused to give in.
However, when he's asked to actually sign his name, John refuses. The act of putting his name to paper is just too much. By signing his name he would have signed away his soul. Though he would have saved his life, his goodness would've been forever out of his reach. With this final valiant act, John Proctor comes to a kind of peace with himself. He says, "I do think I see some shred of goodness in John Proctor. Not enough to weave a banner with, but white enough to keep it from such dogs" (IV.298).