(from Pajiba.com, a great little article on the making of Alien)

Ridley Scott described a guy crouched down, “wobbling” the actors’ seats (to simulate engine thrust), saying it irritated people and that everyone rolled their eyes. “Every step you make, everyone’s a Doubting Thomas.”

Scott said he “…wondered how many people fall by the wayside because you can’t push your point home and don’t quite get what you want. Nobody respects you later for having been a nice guy and given up—you have to get what you want now, because you’re going to wear what you got. You can be very unpopular on the root, but if you’re right, all is forgiven.”