After you send both ghosts through the white light, Joey and Lauren have a long confab in her apartment. Pay attention to this; it's confusing, but important.

In a nutshell, Joey says the Countess is a medium who has lost her spirit guide and somehow instead become linked to Joseph Mitchell's writing. When he writes about someone, whether he publishes the story or not, she thinks that person is a spirit who needs to move on. When she finds they're not dead, she kills them, thinking that she's saving them. Mitchell doesn't know most of this, but he does know that whoever he writes about dies, and so he's stopped writing for fear that someone else will die.

When Joey and Lauren have finished their conference, go see Mitchell again for a conversation that's less friendly but more informative.

Choose any and all conversation options until you get to "Martyr yourself." The idea here is that the only way to get the Countess to appear is to have Mitchell write about Lauren. Yikes!

After that, just go home and ... wait. Oh, and save your game one last time.