I covered the basics of presentation and showmanship here, but we'll specifically talk about when a villain and a hero verbally spar.
First, if the hero isn't doing much except letting you talk, then he's not actively working to stop you. Sure, he might be stalling for time, thinking of a cunning plan, charging up a power, or waiting for the cops to show up, but he's not doing anything to you at the moment.
Second, the hero is the ideal audience on which to test out your theatrics. If you can intimidate a super-powered being, whatever you just said should scare the shorts off of mundanes. Take your "A" material and try it out on a hero. If he's unimpressed, it's probably not the best you can do - or maybe it is, in which case monologues are not for you. Grats, you found that much out at least.
Third, the hero may feel tempted to talk back. If he does, he may inadvertently reveal some useful information. Just be sure that you don't!
The danger of talking while on a caper is that you'll accidentally leak something you didn't mean to talk about. Controlling this tendency during your monologue is vital! If you can't, shut your mouth and keep villainizing. But if you can, the hero often feels compelled to respond in time, and that presents an opportunity to learn something interesting.