RESET A DIALOG
If you are working with any dialog that has an OK and a Cancel option, you don't always have to cancel out to get back to the original state of the effect. If you press-and-hold the Option (PC: Alt) key when you are in a dialog, the Cancel button will turn into a Reset button. This will give you a chance to try the effect again without having to leave the dialog.
To turn on/off preview press P key.