In the scene in Alien when the alien comes out of John Hurt’s stomach, the actors were not actually told it was going to happen. It shows.
Moreover Sigourney Weaver’s role in Alien (Ripley) was actually a man’s role in the book! Inspirational casting! In the book Ripley is a young man forced into leading the ship and given no respect by the rest of the crew – casting Weaver added to that in that the crew don’t respect her due to gender.
It’s a lead I’ve followed time and time again check out my Hamlet (www.hamletmovie.co.uk) where there are numerous male roles played by women to great effect!