Service on this flight and the initial flight from JFK to Prague was excellent. Not one complaint about the flight attendants, they were lovely people and did everything to see to passenger comfort. The problem was that our pilot, upon hitting turbulence over the Atlantic between Greenland and Iceland, chose that moment to get on the PA and announce that it was at each passengers risk that they left their seat, and just at the moment of this broadcast, as the plane was hit by a pocket of turbulence, the pilot screamed into the microphone and cut the transmission. To say that this was distressing is a huge understatement. Upon landing, the pilot did not come out of the cockpit. Perhaps understandably so.