My favorite scene of his, in terms of being extremely funny, is when Sellers appears to Humbert at a hotel and claims to be part of the police officers convention which is also staying there. Sellers is dynamite in this scene in how flawless his timing is as he delivers his long line of oddly stringed together words as a man, just a normal guy who just wants to talk about normal world events to another guy with a normal looking face. His hastened manner of speaking which seems only to become faster as the scene goes on is so weirdly spellbinding as well as so humorous. Then there is of course the rest of the opening sequence which again is comedic magic for Sellers as he plays off so well against Mason's intense performance. Just about everything Sellers does is worth a laugh as he goes about talking and keeps changing subjects, while Humbert is obviously quite focused on a single topic. Almost every second of Sellers's performance here is pure joy when looked upon in the simplest sense, as Sellers is indeed as funny as you'd expect him to be as these various strange character. Of course Sellers is only playing one character, and his intent isn't quite so simple.
Every time we Sellers he is playing Clare Quilty. Now this is not a secret since we are properly introduced to Quilty and everything. This is in plain sight as he evens dons the disguises yet it is quite obvious it's Sellers to us, though Humbert is completely in the dark about that as well as what Quilty's purposes are. Well the truth about Quilty is that he is quite interested in, just as Humbert is interested in, Humbert's step-daughter the titular Lolita (Sue Lyon). Of course by interested in I mean in one of the worst possible ways an older man could be interested in a teenage girl. With this in mind Sellers's work adds more than a few layers past just already being hilarious. The scenes where Quilty disguises himself are not just Quilty being strange, but rather Quilty actively manipulating Humbert for his own ends. Sellers in these scenes actually has this definite incisiveness through his eyes and his words as he's trying to scare Humbert into taking a certain action as well as seems to be trolling him just a bit. There's a more than a little sadistic glee underlying these scenes that Sellers realizes in particularly effective fashion as he crafts Quilty into far more than a comedic distraction building towards when the true purpose of Quilty is revealed within the story.
Now Quilty has other appearances throughout the film, often quite briefly as you might just see him hidden in the background or to the side of the center of the frame. Sellers's exceptional as his demeanor in these moments develops Quilty as not only an enigmatic figure, but also almost has a demonic like presence as he stalks Humbert and Lolita through every move. There is something particularly remarkable as Sellers is so off-putting, and no not exactly in the way you'd expect, even through only his voice when Quilty makes a rather accusatory phone call to Humbert. It's interesting in that Sellers actually in a way confuses the viewer as much as Quilty confuses Humbert. The reason being we kind of know who Quilty is since the first scene of the film is when Humbert goes over to murder him. In this scene Sellers shows who Quilty is when he's not on task as stalker so to speak, which is just hedonistic creep. Sellers does not shy away from that fact in least yet is outstanding since he actually managed to shift the perspective of Quilty throughout the film through his portrayal of Quilty's various disguises. This is amazing work as Sellers can't even pigeonhole himself here. This is indeed a great comedic turn, yet he's equally disturbing in the role as well. He even manages to make Quilty slightly sympathetic in the opening scene by subtly portraying some honest fear and unease in Quilty as he goes about mocking Humbert while knowing death is not far away. None of this really should work but all of it does because of Sellers's fantastic performance.