"I'm pretty ruthless about that; I think when you sign over your story, you sign over your story"

Rachel Griffiths is emphasizing the importance of maintaining ownership of one's story. She believes that when somebody signs over their story, they are giving up control of it. She is recommending that a person need to be very mindful when choosing to give up control of their story, and that they need to be "callous" in their decision-making. This might indicate that a person need to be very selective in who they choose to work together with, and that they ought to be sure to secure their story and their rights to it. Griffiths is recommending that a person should be very mindful of the choices they make when it pertains to their story, which they ought to be sure to secure their rights to it.