I just had a really random thought about how to do a movie with Harvey Dent. For everyone else- have one version of the script. For the actor doing Harvey post-acid- have a script with two versions of each scene- one for if the coin lands good, and one for when the coin lands bad. Make him learn both, and then only when you start rehearsing with other actors do you tell him which is the 'real' version. Make him think about how both options are real and viable and how Two-Face encompasses both outcomes.

Obviously, the actor would hate you. But as a geek, it would be so damned cool.