The voice and mannerisms that Zach Galifianakis uses for Dale Baskets is the same he's used for his own fictional twin brother, Seth Galifianakis. "Seth" has been a staple of Galifianakis' stand-up for years.
Zach Galifianakis originally wanted to cast Brenda Blethyn as Mrs. Baskets, but she wasn't available.