* Ben Stiller as Alex Rose
* Drew Barrymore as Nancy Kendricks
* Michelle Monaghan as Jennifer Connelly
* Harvey Fierstein as Earl
* Eileen Essell as Mrs. Connelly
* Justin Theroux as Michael Connelly
* Robert Smigel as The Voice
The film also features a number of supporting actors and actresses, including:
* Kathy Najimy as Barbara
* J.K. Simmons as Mr. Connelly
* Richard Kind as Mr. Kendricks
* Marisa Tomei as Mrs. Kendricks
* David Cross as Mr. Rose
It's important to note that "Duplex" (2003) is a different movie from "Duplex" (2008). The 2008 film is a horror-comedy about a couple who find themselves terrorized by their new neighbors.