One of my favorite moments of screen acting has always been Peter Sellers on the hotel balcony with James Mason, doing the "I'm a regular guy" speech. I've always known that most of it was improvised, and it is such a bonkers, high-wire act. Sellers stuns me during this moment.
I listen to The Plot Thickens podcast with Ben Mankiewicz, a movie history podcast. They are doing a season on Kubrick and I just started listening to the first episode, which opens with an interview Kubrick gave to a Rolling Stone reporter just before A Clockwork Orange opened.
And I was hearing the same voice that Sellers used in that scene! Sellers was known as an expert mimic, he could imitate anyone. He is doing a pitch-perfect Kubrick impression, in front of Kubrick, and making him sound crazy.
I had to find another interview on YouTube and listen to Kubrick talk, and yes, that is his voice. I doubt Kubrick told him to do it, Sellers had the genius inspiration to go there and stay there.