The characters are mysterious to me. When these two people meet, as different ends of life as they are, they each has a complete chaos or confusion going on in their lives. That's what they have in common, and they recognize that in each other. When Alex comes to Sarah and says he can't do this this thing that they haven't done only then did it feel that it was something she didn't want to lose. I can't point to any single healthy reason for her actions other than my sense that she really did like him.
Every actor seems perfect for their role.
Sometimes when a writer writes a script you have actors in mind, because it helps you picture the characters. In Panic I didn't. I had to sit down and really about who would be terrific for these parts. It wasn't easy. After a lot of thinking, I came up with basically these people.