(written from a Production point of view)
Cassidy is the son of stuntman and stunt coordinator Michael Cassidy and the brother of stunt performers Mickey Cassidy and Mariah Cassidy. He got his Screen Actors Guild card at the age of 6, while working on the 1990 action film Eve of Desctruction, which also featured Tim Russ and Tom Morga. Cassidy was part of the "Best Ensemble Cast" of Short Cuts, which won a Venice Film Festival Award in 1993 and a Special Golden Globe Award in 1994.
Cassidy went on to perform stunts and portray supporting characters in such films as the action comedy Kindergarten Cop (1990), the comedy sequel Problem Child 2 (1991, with Zach Grenier and Elaine and Diane Klimaszewski), Robert Altman's Short Cuts (1993, with Bruce Davison, Robert DoQui, and the late Charles Rocket), the mystery thriller Silent Fall (1994, along stuntman Kim Robert Koscki), the horror film Pinocchio's Revenge (1996, with Larry Cedar, Rosalind Allen, Ron Canada, and Aaron Lustig), and the television sequel Casper: A Spirited Beginning (1997, with Michael McKean and stunts by Chris Blackwood, Gary Guercio, and Cole McKay).
Further feature film performances include the drama Mockingbird Don't Sing (2001, with Kim Darby), the mystery comedy Holes (2003, with Alex Daniels, Rick Worthy, Michael Cavanaugh, Eric Pierpoint, Jeff Ricketts, Jim Wilkey, Eddie Mathews, and Brian J. Williams), the comedy sequel American Wedding (2003), Bad New Bears (2005, with stunts by Webster Whinery, Jr. and Zach Duhame), the comedy The Benchwarmers (2006), and the crime drama Heavens Fall (2006, with Bill Smitrovich and stunt coordination by Kim Robert Koscki).
Cassidy has also portrayed stunt roles in several television series such as Baywatch, Tucker (2001), Drake & Josh (2004, stunt coordination by Vince Deadrick, Jr.), Zoey 101 (2006, along Shawn Crowder, Vince Deadrick, Jr., Cody Klop, and Nancy Thurston), and Ned's Declassified School Survival Guide (2006).