Ros and Guil may be at the center of the action in Rosencrantz and Guildenstern are Dead, but they certainly don't drive it. It can be seen most clearly in Act II how they are just left to sit around and wait unless someone else crosses the stage or tells them what to do. Another main character, the Player, seems to suggest that they should be more active and that Guil shouldn't waste so much time questioning things, but Guil is less concerned with action than with freedom of action. Yet, in the end, the fact that Ros and Guil betray their friend Hamlet makes their passivity morally significant; their failure to act may play a role in their own fates.
The tragedy of the play results not from Ros and Guil's passivity, but from their desire to take an active role in events. If they were consistently inactive, then the play would be a comedy and not a tragedy.