Shepard first made a professional reputation as a "field agent" on the reality series Punk'd. While it wasn't his first onscreen role, Punk'd provided Shepard with the recognition needed to further his onscreen career, and just a year after debuting with Ashton Kutcher's band of pranksters, he was scheduled to appear in three major film releases.
Shepard had a minor part in the 1998 romantic comedy Hairshirt, which was his first film role. In 2004, Shepard appeared opposite Seth Green and Matthew Lillard in the wide-release comedy Without a Paddle, with supporting roles in Sledge: The Untold Story and Mike Judge's science fiction comedy Idiocracy following soon thereafter. He performed small-screen work on My Name Is Earl and Robot Chicken shortly before climbing into astronaut gear for Jon Favreau's 2005 fantasy Zathura.
In 2006, Shepard appeared opposite Dane Cook and Jessica Simpson in the comedy Employee of the Month. During the same time, Shepard began appearing in more film roles, and starred his first main character role in Let's Go to Prison. He also had a major role in Baby Mama. Shepard wrote the script for the Paramount venture Get 'em Wet, in which he again appeared opposite Arnett.