Stepanek is best known for his role as Arwin in the Disney Channel sitcom The Suite Life of Zack and Cody. He also guest starred in an episode of The Suite Life on Deck, It's All Greek to Me as Arwin and Milos, and in CSI: Miami, Gandu Three Echo in the film The Island and a Sector Seven Agent in Transformers, The West Wing, The Drew Carey Show, Murder By Numbers, NYPD Blue and JAG.
Stepanek has also done additional voiceovers in the movie Lemony Snicket's A Series of Unfortunate Events, Kim Possible in 2007 and the films Charlotte’s Web and Over the Hedge. Another appearance is as a strange chef named Lowell in What I Like About You and the sit-com, Malcolm In The Middle. He was also one of the co-hosts for the first ever Disney Channel Games. He played the Riddler in one ad from the Batman OnStar commercials.
He portrayed one of two Caucasian cops in Friday After Next. He is well known as "The Dex Guy" Dex, and has also done commercials for Ace Hardware, Advil, Toyota, and Dairy Queen as well as radio commercials for Microsoft.
In 2008, he played as Martin on the movie Bolt. He was also an uncredited policeman in Friday After Next, where he took Marijuana for "testing". He also has his first voice role in The Secret Saturdays as Agent Epsilon.
Stepanek currently stars in a 5-minute short show called Brian O'Brian, a silent slapstick-style show aired on Disney Channel. He also stars in the Disney Channel Original Movie Hatching Pete as the Coach Mackay, and Mostly Ghostly: Who Let the Ghosts Out? as Phears.