There are various instances in Romeo and Juliet where the acting of the characters turns out to be their fate. Mercutio jokes around in his sword fight with Tybalt and when Tybalt stabs him he tries to be serious telling everyone that he is hurt, but at the same time he kids around by saying, "tomorrow you shall find me a grave man."
He asks Benevolio to get him inside a house but everyone laughs at him because he is never serious. This acting or "clowning around" led to his demise.
Then Juliet takes a potion that is supposed to simulate death. Romeo finds her in the tomb only to think she is actually dead and thus not wanting to live without her, drinks poison. Then Juliet awakes to find Romeo dead and she stabs herself. What was actually acting turns out to be their fates.