If you like plane-spotting and enjoy just watching the AI aircraft taxi-ing, taking off and landing and just generally experiencing the airport’s ambience without actually flying, don’t park on a runway or begin a flight from the end of a runway. If you do, X-Plane will act in a logical manner and wait for you to take off. AI activity will come to a stop while X-plane waits for you. If you are in the “ready for takeoff” position you may well find that an aircraft creeps up behind you and just stops and waits. Or sometimes all AI activity just simply stops.
If you want to watch aircraft, rather park well off a runway, or even better, simply stay at your gate, press the 
key for free camera movement, and “walk” about using your keyboard and right-mouse button.