If your world is divided into a pretty objective good and evil and the hero becomes a villain by moving from good to evil then this is more difficult, because it's a big (and therefore improbable) change in personality. So it's pretty tough not to make it seem contrived, and not to completely repeat a cliche.
If the people in your world are like people in the real world, they pretty much all think of themselves as the heroes of their own story, and only play the role of the villain in the perception of others. So they may not have to change personality at all to become villains, they may just need their circumstances to change so that others perceive them that way.
For example (similar to JohnP's example, though I thought of it before I read his answer), a hero might be an idealistic, charismatic leader that overthrows a tyrant. But he isn't a good ruler. Maybe he trusts those who helped him overthrow the tyrant too much, even though some of them have happily helped themselves to the rewards of power. Now he's been forced to use some of the tactics the former tyrant used to keep power, because he fears the next ruler will be worse than he has had to become. He fights against the PCs with everything he has because he thinks they will be part of that.
You asked for a theatrically rich way to stage this, so how about if the Hero's tragic flaw that makes him into a villain is the same thing that made him a hero? In many tragedies the fatal flaw is just the other side of a positive trait essential to the character, and had the hero been destined for a kinder fate that dark side might never have mattered. Othello might have gotten away with loving not wisely but too well if not for Iago, Romeo and Juliette's youth and impetuousness were essential to what made them "heroic" but need not have lead to their death.
So you can pretty much think of whatever heroic traits fit your campaign, and think about how they could go bad (without really changing the hero). Loyalty could be to the wrong person or cause. Bravery can be foolish, dragging others down with you. Virtuous can become judgmental, or hidebound and unable to adapt. Generosity can be misplaced, or ruinously excessive, or require going to far to attain things to be generous with. Refusing to let the ends justify the means could lead to the hero fighting desperately against those (such as player characters) trying to avert tragedy.
To quote from Hamlet:
But, orderly to end where I begun,
Our wills and fates do so contrary run
That our devices still are overthrown;
Our thoughts are ours, their ends none of our own