What will be the output voltage when 9 volt given to LM7812
It's not quite as simple as a fixed dropout voltage. 

Here is a plot of the dropout characteristics of the LM340 which is sold as the same part as the LM78xx series. This is a 5V part so imagine the graph extended up to +12 rather than 5. 



Nothing very exciting there, though you can see the drop decreases from 1V at 3V in to about 0.5V at 4.5V in (with zero current draw), and below about 2.75V the part has virtually no output as the internal biasing fails to keep the 'lights on'.  A fixed dropout voltage would have a slope of 1.00 and extend all the way to the 'X' axis. 

The quiescent current is well-behaved: 



There was at least one older bipolar regulator (Low dropout = LDO type) that had Iq spiking near dropout as the internal circuit tried to maintain regulation by sucking lots of current through the (very low gain) lateral PNP pass transistor base. 



Keep in mind that about 3V headroom is required to keep the regulator happy with 1A flowing under all conditions- so the valleys in any input ripple on a 7812 should not go lower than about 15V. 



Applying a negative voltage to the regulator input relative to the GND pin, if more than a diode drop will cause a lot of current to flow and can easily destroy the regulator.