It is clear from the outset that both Abigail Williams and John Proctor are leaders. Abigail is the one who leads the activities in the forest before being discovered by her uncle, the Reverend Samuel Parris. She is also the one who takes Tituba's lead in identifying so-called witches and leads the girls into repeatedly putting on an act to further mislead the court. It is clear that the other girls fear her, for she threatens them 'a pointy reckoning' if they should testify against her, or tell the truth.
Abigail is typical of a rebellious teenager, who, when caught in circumstances which stifle her freedom, will fight against them and thus break the rules. She knew fully well that what she was doing in the forest was wrong, but she did it anyway. Abigail had been corrupted by John Proctor, who 'put knowledge in her heart' by conducting an adulterous affair with her. Her innocence had been tainted and she obviously believed that John would turn to her. It is this, and her fear of being branded a witch, that brought her malevolent nature to the fore.
In her desperate desire to protect herself, Abigail plotted and used deceit to implicate others. Of course, being a teenager, she also relished the attention she received when placed on a pedestal - she was the lead witness during the trials and people parted 'like the waters of Israel' where she walked. Furthermore, the trials gave her much power and this was her opportunity to get back at those who she felt had done her wrong and express her vengeful nature.
It also gave her a chance to take vengeance on Elizabeth for dismissing her and, once she was out of the way, she could reclaim John who she knew still had a soft spot for her. It is pertinently clear from her actions throughout the play that Abigail is not only implicitly evil and remorseless, but that she also has other criminal tendencies, as displayed when she steals her uncle's money before secretly leaving Salem. One can only surmise whether Abigail would have been different if she had not had the affair with John Proctor.
John Proctor is a forthright, confident man who is severely critical of those who he believes are failing in their duties or who are corrupt. He is also very direct, and has no qualms in saying what he feels, no matter what the consequences. He is tortured by his sin, for he is, in essence, a good man. He bitterly resents Elizabeth's continuous suspicions about him and asserts that he will not commit an act of deceit, for he is an honest man.
However, John is also not always entirely honest. He for example, lied to Elizabeth about his conversation with Abigail, telling her initially that he had spoken to her when they were in company, when they had actually been alone. Furthermore, he refuses to acknowledge that he has at least some feeling for Abigail. This makes Elizabeth resentful for she feels that Abigail still 'has an arrow' in him.
John is deeply loyal and even in the most desperate circumstances, refuses to betray others. When he is asked to name those with whom he had supposedly consorted with the devil, he vehemently resists, saying that they had found their salvation and no one can expect him to take that away from them. It is also evident that he is deeply aware of the importance of retaining his good name. When he realises that the respect the community has for him, would be abused, he refuses to hand over his written confession for he wants to die with his integrity intact.
It is also clear from brief references throughout the play that he is dedicated and hardworking, a devoted father who has great love for his children and deep affection for Elizabeth, despite his affair with Abigail. The two eventually resolve their differences in the dramatic final moments of the play.
John Proctor and Abigail Williams have a few traits in common. They are both strong-willed, stubborn and persuasive. This is where the similarities end, however.
Though Proctor was dishonest enough to have an affair with Abigail, cheating on his wife, he has repented. He is, for the most part, dedicated to honesty. He is a man concerned with integrity (a fact demonstrated by his deep worry that he has lost a chance to regain it). We see this part of his character very clearly in the final scenes of the play, when Proctor is jailed and facing execution. His speeches in this final part of the play are largely concerned with ideas of honesty and integrity.
Abigail has no such concern. She is deceptive, manipulative and selfish. She evades responsibility for the mistakes she has made - and she has made quite a few. Proctor, in contrast, confesses to his mistakes both privately and publicly.
Abigail's penchant for escape is nicely encapsulated by the fact that she literally runs away in the end.
In addition to e-martin's answer, we may observe that Abigail is not merely selfish and manipulative: she is also dedicated to vengeance. After Proctor rejects her she turns vehemently against him. She deliberately drags Elizabeth down, not just accusing her of witchcraft but contriving to plant evidence against her (the poppet). Of course, she regards Elizabeth, wife of her former lover Proctor, as her chief enemy. When Proctor rushes to the defence of his wife he is inevitably sucked into the whole maelstrom as well, and eventually hanged. Abigail thus gains her revenge on the two of them.
However, it is possible to go too far in condemning Abigail for her actions. Although she certainly is lying and vengeful, we should remember that she is still quite young and alone in the world (apparently she witnessed the murder of her own parents). We see her vulnerability in her first scene with Proctor when she pleads for his love, as he gently but firmly breaks off their relationship. Rebuffed in this manner, it is little wonder that she is so crushed, although it does not excuse her subsequent behaviour it goes some way towards explaining it. Proctor is considerably older than she is and should really bear the chief responsibility for their affair. Of course, he feels terribly guilty afterwards, but he seems to just expect Abigail just to shrug off the fact that he has spurned her because it would be convenient for him. Thereafter he is quick to characterize her as a wicked whore. He does not act wisely in relation to Abigail and brings down retribution on his own head.