The character he plays is a creepy little perve. Boy blackmails girl, boy loses girl, boy gets girl back. I'm never sure whether films and plays about misogynists are necessarily misogynist in themselves, but I guess this one probably is. Sellers gets to say things like "Every pretty girl is like a flower garden with a compost heap at the bottom" and sing parlour ballads about wife murder.
Initially he wanted to play the character with an Austrian accent and lots of pratfalls. But the director challenged him to do it straight. So he did. The outcome is something very like a self portrait. The void shows through.
I've read the Life and I know what a swine he was, but Sellers is one of the very few actors I really love.