Quilty on drugs?

Yes. It seemed utterly manic to me. And honestly, it seemed like someone on the autism spectrum.

I'd be surprised if the character didn't do drugs.

I thought he was just crazy with guilt and the fear he'd been caught, and would lose Lolita. He was supposed to be going insane from the emotions. And he had been drinking a lot to put up with Catherine, whom he did not love, and maybe hated. He'd just driven a long way, lied a lot...then he thought the cop saw right through him. And he had not had a drink on the road probably...so he was very shakey.

Well considering Kubrick allowed Sellers to ad-lib much of his dialogue, and Sellers experimented with various drugs during the years, that would not be beyond the pale! Many that worked with Sellers through the years, also observed that his actions could be very manic at times, though he refused any psycological assistance. That could all add up to the strangeness that was Clare Quilty!

He seems like the type that could be found coked out of his mind.

Movie girl: It did not strike me that he was on drugs, but was trying to be humorous (in an inane fashion) and pretending to be nice. Even at this time Quilty suspects Humbert of an affair with Lolita? He seems to trail him everywhere - even with his girlfriend at the party. Funny that Humbert is stressed out and does not realize that this is the same guy.

It was just the acting and the horrible character that sellers made up . He was the one weak link in the film .