Playboy:

Knife in the Water was an original, and unusual screenplay. Where did you get the idea for it?

Polanski:

It was the sum of several desires in me. I loved the lake area in Poland and I thought it would make a great setting for a film. I was thinking of a film with a limited number of people in it as a form of challenge. I hadn't ever seen a film with only three characters, where no one else even appeared in the background. The challenge was to make it in a way that the audience wouldn't be aware of the fact that no one else had appeared in the background. As for the idea, all I had in mind when I began the script was a scene where two men were on a sailboat and one fell overboard. But that was a starting-point, would you agree?

Playboy: 

Certainly, but a strange one. Why were you thinking about a man falling out of a sailboat?

Polanski:

There you go, asking me how to shrink my head again. I don't know why. I was interested in creating a mood, an atmosphere, and after the film came out, a lot of critics found all sorts of symbols and hidden meanings in it that I hadn't even thought of. It made me sick.

(Playboy, December, 1971)