What important function does Jocasta play in the play and the film other than being Oedipus mother/wife?
Aside from the fact that Jocasta is Oedipus' mother, wife, and the mother of his children, she plays a role in the drama unlike the rest of the characters. In the midst of his trials, Jocasta is the one voice that seems to support Oedipus through reason. She speaks very matter of factly in Act II when she explains to Oedipus why she does not believe in prophesy. Unlike the rest of the people in Thebes (and therefore also the original audience of the play), Jocasta dares verbalize her disbelief in something everyone else takes very seriously and without question.
Ironically, while Jocasta's confidence makes her sound very rational and even somewhat convincing, her actual proof for why she does not believe in prophesy (the story of the crossroads where Laius was murdered) turns out to be the exact evidence that confirms Oedipus' mistake and shows him his own blindness. She unknowingly convinces him that Tieresias was correct all along.
When she hangs herself at the end of the play, I think it symbolizes the fact that Jocasta really thought herself innocent all along. I believe it shows that her intentions were pure and only confirms her ignorance. When she finds out the entire truth, and realizes how her mistakes contributed to the tragedy, her guilt is so great that she does not feel worthy to live. There is something noble in this decision, despite how much the audience may pity her.