"Do I really look like a guy with a plan?" Actually, yes, given everything we've seen you through, you are most definitely a manipulative guy with a plan. If only Harvey were in a stable enough mental state to come to that conclusion. He was so brilliant!
The brilliant Joker never gave Harvey a choice at all, which is a nice scripting of "the illusion of free choice" making The Dark Knight undeniably the best film in the trilogy. Upon looking at his hand placement, Heath Ledger's Joker would've kept the hammer from falling even if Harvey pulled the trigger.
If Harvey's coin had landed on tails, and it probably did, he would've shot, then enter the big speech about chaos and the illusion of choice, Joker throws him the gun, and walks out unscathed. Isn't that crazy? Why do we know he probably did shoot? Harvey had to be "broken" to go on his little spree of revenge. Coin lands on tails, for the first time he lets himself willingly commit murder, but plot twist, the Joker ends up living, and Harvey is now taught to be capable of murdering someone if he first gives them a fair chance.
Other cool facts: Heath Ledger designed the make up himself from white face paint and drugstore cosmetics, and Michael Caine was too startled by Ledger's appearance to say his lines during their first appearance together at the fund raiser. If there was a re-shoot, Christopher Nolan likely decided to keep the silent startled reaction.