The absence of free will
Every time Rosencrantz and Guildenstern consider making a real choice, something stops them or they simply drift back into passivity. The play keeps asking whether they could have done anything differently, and keeps refusing to answer. That refusal is the argument.
Identity and interchangeability
The two men blur into each other. They swap names, forget which is which, and are treated as a unit by everyone at Elsinore. Stoppard uses their indistinguishability to show what happens to people who never define themselves against the world around them.
Death and meaninglessness
Death in this play isn't dramatic. It's an absence. The Player treats it as theater. Rosencrantz and Guildenstern treat it as an abstract puzzle. When their own deaths arrive, they're not even staged. Stoppard makes the point that death without meaning is just disappearance.
Performance and reality
The play-within-a-play structure keeps blurring the line between acting and living. The Player performs death convincingly enough to fool everyone. Rosencrantz and Guildenstern perform their assigned roles badly. The question of what's real and what's staged runs through every scene.
Being a minor character in someone else's story
Hamlet's plot moves forward whether or not Rosencrantz and Guildenstern understand it. They're pulled along by events they can't see clearly. Stoppard takes this structural fact and turns it into a philosophical condition: what does it feel like to live in a story that was never about you?