Rebecca Nurse is a woman of good character, honesty, and moral integrity. When she is accused, this should have been a moment for the people of Salem to give pause and reconsider the accusations. Rebecca Nurse will not go along with the hysteria. And despite the fact that it might save her life, she refuses to sign the confession.
John Proctor is a bit more conflicted than Rebecca and a bit more flawed, considering his affair with Abigail. However, overlooking that mistake, John is also generally honest and he is brave enough to admit that he has never trusted Reverend Parris' judgment or intentions. Rebecca's refusal to confess is one factor that moves John to reconsider and also refuse to sign the confession.
Both Rebecca and John stick to their convictions. John just has a momentary lapse of judgment.
Other characters in The Crucible, Elizabeth, Giles Corey, and eventually Hale, see that the witch hunt is false. However, most of the other people in Salem go along with the hysteria either because they believe in it or because it gives them an opportunity to accuse others for personal reasons. For example, Thomas Putnam sees an opportunity to buy land from the accused at lower rates.
Another thing is that Rebecca and John understand that their leaders are corrupt and that the witch hunt is a moral tragedy. They declare that they only answer to God and that is enough. At the end of Act 4, when John confesses and tries to keep the paper confession, he claims they don't need the signed confession any more:
I have confessed myself! Is there no good penitence but it be public? God does not need my name nailed upon the church! God sees my name; God knows how black my sins are! It is enough!
Shortly after, when John tears up his confession, Rebecca approves of John's defiance of Danforth and claims that the moral and honest judgment of God awaits everyone. To John, she says:
Let you fear nothing! Another judgment waits us all!