Lets us say if hijackers have entered to an aircraft which was ready for the departure, if someone(pilots/flight attendants or passengers on board) got a chance to evade from the aircraft who should evade first? Is there any operational procedure to follow during such an incident?
I know that this is somewhat a weird question, but I got this problem when watching a movie called Neerja
about the Pan Am flight 73.
I have no special insights, but logically I think the answer is no one should leave the plane until a police response.
If the hijackers are known to the pilots before before departure the plane shouldn't take off. The pilots are in a more or less bulletproof room and they have the final say if an airplane takes off or not. They also have access to radios that can request a police response to the airplane, in addition to something more mundane, like parking an ARFF vehicle in the way of the aircraft's movement.
If the pilots aren't informed of the hijackers, but the FAs know of the hijackers? In this case, the FAs could do something subtle, like not responding to the pilot's requests, or opening an exterior door, which would be noted in the cockpit. This would then result in pilots taking action to address the unknown issue. There are also are systems where the cockpit has video cameras into the cabin, so they could use this tool in determining what the issue is. (FWIW, this is something LiveTV has advertised on their website.)
The passengers shouldn't be evacuated before a police response, because the hijackers will blend in with the passengers in the chaos, and depending on the location of the plane on the airfield could escape off of airport property.
All of these presuppose a post-September 11th hijacking, where the aircraft itself is presumed to be a weapon, and it is assumed that in any scenario everyone onboard will be killed. In that case, it is better that only the people on the airplane be killed, instead of everyone on the airplane being killed in addition to those on the ground being killed as well.
In a pre-September 11th hijacking, I believe the scenario that was generally followed was one of cooperation, but that was only after the airplane was in the air. That being said, there were many times an hijacked airplane stopped for refueling.