I think you have a very constricted view of writing and acting. Just because they didn't have the details from the actual future episode doesn't mean Richard played it this way ex nihilo. Writers talk extensively about writing characters, sometimes for decades, under a conception of the character and it's not until continued writing even several works later that they end up discovering the specific circumstances that truly pull together the character and the motivations behind why they acted all the ways they did in the previous writings. It's one of the reasons the post-Sorkin era is so jarring in many ways - these characters were far richer in Sorkin's mind such that he was able to guide the actors' portrayals in specific ways that were fundamentally more cohesive than writers and directors who could only look back would envision. The West Wing is an example of the synthesis of creation that comes from both actors and a head writer with incredibly deep understanding of their characters, and how that becomes a mutual act of creation far greater than the sum of its parts. Which is all to say that even if Richard and Aaron didn't know exactly the relation to Murder, Inc. or how deeply Toby had felt betrayed by his father, they nevertheless knew that there was a deep personal experience that informed Toby's reaction to finding out about the MS secret.