"Can I borrow the hair dryer?"
"What on earth for?"
Just tested. If the inside of your windshield is frosted over, the hair dryer technique works much faster than idling the car and running its internal heater.
As a bonus, the hair dryer, if aimed like above, gets rid of the frost from the top of the windshield. The car's heater gets rid of the frost from the bottom, slowing down the defrosting even more. A couple of minutes with the dryer, on high heat and full power, saves ~10 minutes in idle waiting for the heater to do its thing.
(Or, save you from slouching and peeking through the defrosted bottom of the window....)
