You might find it useful to see how Miller introduces these two characters to gain some sense of their characteristics and how they can be compared and contrasted. For example, Miller introduces John Proctor in the following way:
He need not have been a partisan of any faction in the town, but there is evidence to suggest that he had a sharp and biting way with hypocrites. He was the kind of man - powerful of body, even-tempered, and not easily led - who cannot refuse support to partisans without drawing their deepest resentment. In Proctor's presence a fool felt his foolishness instantly - and a Proctor is always marked for calumny therefore.
... He is a sinner, a sinner not only against the moral fashion of the time, but against his own vision of decent conduct.... Proctor, respected and even feared in Salem, has come to regard himself as a kind of fraud.
This clearly paints a very useful picture of John Proctor at the start of the play, and you might want to pick the three following adjectives out to describe John Proctor: respected, feared, and troubled.
Rebecca of course, although she ends up sharing the same fate as John Proctor, is a very different character. Consider how Miller presents her:
As for Rebecca herself, the general opinion of her character was so high that to explain how anyone dared cry her out for a witch - and more, how adults could bring themselves to lay hands on her - we must look to the fields and boundaries of that time.
Note how this emphasises her goodness and standing in the community by saying that it was only the rapaciousness of the Putnams that ever allowed Rebecca to be arrested. You might also want to look at how Rebecca Nurse calms Betty:
Rebecca walks across the room to the bed. Gentleness exudes from her.
Thus we might want to use the following adjectives to describe Rebecca: respected, gentle, saintly.
It is interesting to note, lastly, that Rebecca never tries to argue against or toys with the idea of going against being hung, unlike John Proctor - she is set in her path and meets it with a God-given sense of peace and calm, unlike John Proctor, who is able to meet his fate only after a time of doubting and changing his mind.