25 Things You Didn't Know About Alien
22. The Alien's Acidic Blood Was An Afterthought During Writing
One aspect of the alien's design that makes it so memorable, iconic and deadly is its highly acidic blood, one small spillage of which manages to melt through two floors of The Nostromo. However, the script was mostly complete before this detail was added.
Screenwriter Dan O'Bannon, when revising the script, was struggling to find a reason that the crew wouldn't just shoot the alien with a gun (though surely the fact that The Nostromo is a freight vessel with no guns on board would suffice).
It was conceptual artist Ron Cobb who suggested the acidic blood, as retaliation would put the crew in jeopardy - and thus one of the alien's greatest features was born.