I'm going to try at any rate. First of all, a capacitor is simply 2 plates separated by an insulator.
Well, consider a piece of 2 conductor cable. What do we have? 2 charged bodies separated by an insulator - a capacitor! It should be noted that wire has extrememly small amounts of capacitance (and inductance too) at 60 hertz.
Now, the story changes entirely when we talk about high frequencies. A short piece of wire can turn into an open circuit at high frequncies due to capacitance, but that's a whole 'nother story. Let's just say I wouldn't want to be in integrated circuit design right now.
So, in the case of this 12/3 with one of the two hots deenergized, the live conductor actually induces a voltage on the other dead conductor due to capacitance. This is where I get lost in the physics of it all, so hopefully someone else can jump in.