OK, I read it on Facebook, but don't know whether it is true or not. The scene where the Joker blows Gotham Hospital up was initially not according to plan, but Heath Ledger improvised with his skills. So, initially when he presses the button a very small explosion happens. Then we can observe that Heath Ledger gets confused (which does not look like he is acting to some extent to me) & then presses the remote multiple times. Then suddenly the whole thing blows up, Heath is definitely taken for surprise here.
So, just wanted to know whether there is any proof whether all this was part of the script or was it a goof-up covered up by Heath?