Here's an interesting fact about the making of Das Boot, an article taken from Wikipedia and verified by other sites also:-
A mock-up of a conning tower was placed in a water tank at the
Bavaria Studios in
Munich for outdoor scenes not requiring a full view of the boat's exterior. When filming on the outdoor mockup or the conning tower, jets of cold water were hosed over the actors to simulate the breaking ocean waves. During the filming there is a scene where actor
Jan Fedder (Pilgrim) falls off the bridge while the U-boat is surfaced and lands in the front and breaks several ribs. This scene was not scripted and during the take one of the actors exclaims "
Mann über Bord!" in order to draw attention to Fedder. Petersen, who at first did not realise this was an accident said "Good idea, Jan. We'll do that one more time!". However, since Fedder was genuinely injured and had to be hospitalised, this was the only take available and eventually Petersen kept this scene in the film. In this scene, the pained expression on Fedder's face is authentic and not acted. Petersen also had to rewrite Fedder's character for a portion of the film so that the character was portrayed as bedridden. For his scenes later in the film Fedder had to be brought to and from set from the hospital since he suffered a concussion while filming his accident scene. Fedder eventually recovered enough and Pilgrim is seen on his feet from the scene when the U-96 abandons the British sailors
Now thats hardcore acting!!!
http://en.wikipedia.org/wiki/Das_Boot