I defended my PhD in 2009 at George Mason University (GMU). It had two halves, but they were reversed: the first half was private, the second half was public. I think that's a better system.
In the first half I was privately grilled by my committee. They wouldn't let the dissertation go to a public defense until they'd satisfied themselves that it was fine.
As far as I can tell the public defenses usually look ceremonial at GMU (at least in its engineering department), but they aren't actually ceremonial. Anyone from the public can ask questions at a public defense, so they aren't ceremonial. However, the goal of the (first) private half was to try to make sure that the defender is ready for arbitrary questions (because he understands the material). So it's unusual for the public to ask questions that the defender isn't able to answer. I got some questions I hadn't heard before in my public defense, but I was able to handle them.
In the first half I was privately grilled by my committee. They wouldn't let the dissertation go to a public defense until they'd satisfied themselves that it was fine.
As far as I can tell the public defenses usually look ceremonial at GMU (at least in its engineering department), but they aren't actually ceremonial. Anyone from the public can ask questions at a public defense, so they aren't ceremonial. However, the goal of the (first) private half was to try to make sure that the defender is ready for arbitrary questions (because he understands the material). So it's unusual for the public to ask questions that the defender isn't able to answer. I got some questions I hadn't heard before in my public defense, but I was able to handle them.
You can see my public defense here: https://www.youtube.com/watch?v=QYH18NpsRu8