The mother gets a big one in her speech to Karras, who after examining Regan feels that more treatment is needed, proves that she is a mix of Mama Bear and Determinator.
Chris MacNeil: "You show me Regan's double, same face, same voice, everything. And I'd know it wasn't Regan. I'd know in my gut. And I'm telling you that thing upstairs isn't my daughter. Now, I want you to tell me that you know for a fact that there's nothing wrong with my daughter, except in her mind. You tell me for a fact that an exorcism wouldn't do any good. You tell me that!"
Father Karras, after seeing Father Merrin has died—possibly at the hands of the demon—regains his faith, demands the demon enter him and when he realizes the demon will use him to kill Regan, sacrifices himself by jumping out the window.
Merrin himself is made of awesome. He drops what he's doing in Woodstock just so he can save a poor soul (In this case Regan.) from the demon. As soon as he arrives, he takes control of the situation and mentors Karras giving him advice and even provides a mental anchor when the Demon tries to psyche out the younger inexperienced Karras.