There will be blood - Mary Sunshine

So, I have eventually watched There Will Be Blood, and am wondering about the name of the production accountant, Mary Sunshine. She appears to be a well established production accountant, but to have the same name as one of the films main characters seems more than coincidental. What's the story here?

The character is Mary Sunday.

Oh yes! Well, don't I feel stupid!!

no worries - it's a great movie! I think the best one of the century.