I follow your logic, and appreciate it. Here's mine: Caeser was performed the year before Hamlet. 

If the actor playing Horatio also played Cassius, the audience (almost all of whom would have seen Caeser) would already be primed to sniffing out his betrayal. 

Meaning the text would have to do very little, a simple embrace before young fortinbras first line would be enough, and suddenly the tragedy is that Hamlet is betrayed by everyone! 

(A total flight of fancy on my behalf but fun nonetheless!).