Brett, Charles and Richard knew exactly how to keep the game moving while being spontaneous and funny. Brett and Charles just had that natural chemistry, and Richard had a large enough library of jokes and short stories to balance it out. The sixth chair, being a rotating semi-regular, always had a unique "final statement", and since the game is often decided with the sixth seat, they better have the "definitive" answer.
Once Richard stopped participating, Brett and Charles ended up working harder, and in effect this made their interaction even better and funnier.