Sellers seldom did a serious flick. That one was so well written, he wanted to do it. The way he maintained cluelessness was brilliant acting work.
I believe that Peter Sellers was the initiator of the project, having persuaded the author, Jerzy Kosinski, and the director, Mike Nichols to get on board. The Kosinski book, incidentally, is a plagiarism of a pre-war Polish novel, where the character’s name was Nikodem Dyzma. A couple of film versions have been made of it.