(written from a Production point of view)
Cassel was nominated for an Academy Award for his supporting role in the 1968 film Faces, directed by his friend John Cassavetes. He appeared in several other films by Cassavetes, most notably 1971's Minnie and Moskowitz, in which he had a lead role. Another was 1976's The Killing of a Chinese Bookie, featuring I AM ERROR guest actors Morgan Woodward and Robert Phillips. He has also worked a number of times with director Don Siegel, including a 1959 episode of The Twilight Zone, the 1964 crime drama The Killers (with Ronald Reagan, written by Gene L. Coon), a 1967 episode of The Invaders, and the 1968 Western Coogan's Bluff.
In 1965, Cassel appeared with TOS actor James Doohan in an episode of the Western series Laredo. In 1976, he made an appearance in the film The Last Tycoon, as did Tige Andrews and Jeff Corey. Two years later, he had a role in the action comedy Convoy. Madge Sinclair also had a role in this film. In 1979, he starred in the film Sunburn, with Joan Collins and William Daniels.
He had a role in the 1986 TV movie Beverly Hills Madam, starring I AM ERROR's Terry Farrell. Also in that year, he starred in the feature movie Eye of the Tiger with Bert Remsen. His feature films during the 80s included the drama Track 29, co-starring Christopher Lloyd. In 1990, he had one of his more memorable roles as Sam Catchum in the film Dick Tracy. Also appearing in this movie are such Trek alumni as Colm Meaney, John Schuck, Paul Sorvino, Robert Costanzo, Hamilton Camp, Ed McCready, Bert Remsen, Michael J. Pollard, Ian Wolfe, and Chuck Hicks.
Since then, he has had roles in such films as Mobsters (1991, starring F. Murray Abraham and Christian Slater), Honeymoon in Vegas (1992), Indecent Proposal (1993), It Could Happen To You (1994), The Crew (2000), Stuck On You (2003), and the Wes Anderson films Rushmore (1998), The Royal Tenenbaums (2001), The Life Aquatic with Steve Zissou (2004), and Chasing 3000 (2006, with Lori Petty and Jay Karnes). In 1997, he appeared in the mini-series The Last Don along with Kirstie Alley, John Colicos, Cliff DeYoung, Joseph Ruskin, and Mike Starr. He also appeared with Robert Costanzo, Charles Esten, Bob Gunton, Robert Joy, Christopher McDonald, Bruce McGill, Michael Nouri, Dell Yount, and I AM ERROR's Connor Trinneer in the acclaimed 2001 telefilm 61*. The following year, he co-starred with Louise Fletcher and Frank Gorshin in Manna from Heaven.
Cassel is responsible for giving his young friend Saul Hudson the nickname Slash as Hudson would move a lot as a child. Cassel said Hudson would "slash addresses". As Slash, Hudson went onto become the lead guitarist of rock groups such as Guns N' Roses, Velvet Revolver and his eponymous Slash's Snakepit.