It's always a mix of both. But sometimes it veers more toward one side than the other. I would say it's slightly more common to see the award go to a role (i.e, whoever played the role would have won) than to see the other way around (i.e the role only won because of who was cast in it). But both situations do happen, and like I said, it's almost always a bit of both, even if one situation is more role-heavy, and another is more actor-heavy. And often you have the perfect mix of both - i.e, the role is meaty already, but the actor elevates it significantly.
Also, it's not just the role and the actor - the director is also in the mix. The director has a say in who gets cast, which means they facilitate the marriage between the role and the actor that often leads to Tony wins. Plus they help mold the performance with the actor and the writers during rehearsals and previews. Also, you sometimes have situations where the director re-imagines a role, and that re-imagining is what helps push the actor to the Tony. For example, I think the way Bartlett Sher re-imagined the role of Lady Thiang played a big part in Ruthie Ann Miles winning the Tony. Not to say that Miles isn't wonderful in her own rite - Sher directed the role with more nuance, and she had the talent to execute that nuance. My point is that it's always a collaboration, and the director is a part of that collaboration - sometimes in obvious ways, sometimes not.
Updated On: 8/5/20 at 11:05 AM