(written from a Production point of view)
De Souza guest starred in two episodes of Mission: Impossible while TOS star Leonard Nimoy was a regular on that series. One of the episodes he was in, 1969's "The Brothers", also guest starred Lee Bergere, Lee Duncan, and Joseph Ruskin. He appeared in two episodes of The Man from U.N.C.L.E., one ("The Yellow Scarf Affair" in 1965) with Willard Sage and another ("The Moonglow Affair" in 1966) with Dallas Mitchell, Byron Morrow, and Bill Quinn.
De Souza's other television credits include a 1958 epiisode of Zorro titled "Zorro Fights His Father", with Peter Brocco, a 1959 episode of Adventures in Paradise called "The Derelict", also guest-starring Ricardo Montalban; a 1963 episode of The Eleventh Hour with Richard Kiley and Malachi Throne; a 1963 episode of The Outer Limits with Henry Darrow and Willard Sage; a 1974 episode of Kolchak: The Night Stalker, starring John Fiedler; and a 1989 episode of Tour of Duty, starring Dan Gauthier and Patrick Kilpatrick. He was also seen on the Western series Wagon Train, the sitcom Sanford and Son, and episodes of Bracken's World (1970, with Monte Markham and Madlyn Rhue) and Bring 'Em Back Alive (1982, with Harvey Jason and Eric Server).
Both De Souza and George Takei had uncredited roles in the 1959 war drama Never So Few, which featured Whit Bissell and John Hoyt. De Souza also appeared uncredited along with Vic Tayback in the 1966 comedy Gambit, starring Roger C. Carmel. He had roles in the films Bad Charleston Charlie (1973, with Ken Lynch), The Man with One Red Show (1985, with Gerrit Graham, David L. Lander and David Ogden Stiers), Wildcats (1986, with Bruce McGill and Bruce French), and Deadly Care (1987, with Paddi Edwards).
In 1977, De Souza co-starred with John de Lancie, Persis Khambatta, and Jason Wingreen in the science fiction television movie The Man with the Power. He went on to work with Persis Khambatta again in the 1988 direct-to-video thriller Deadly Intent, co-starring Clayton Rohner. In 1985, De Souza had a role in My Wicked, Wicked Ways... The Legend of Errol Flynn, a television movie starring Duncan Regehr and Denise Crosby. He also appeared in the horror thriller B.O.R.N. (1988, with Clint Howard and Debra Lamb Bailleaux) and the television drama Unabomber: The True Story (1996, with Dean Stockwell, Bill Mondy, and Kevin Rahm).
Following his appearance in Star Trek, De Souza had supporting roles in the comedy Wedding Crashers (2005, with Ellen Albertini Dow, Ron Canada, Henry Gibson, Diora Baird, Irene Roseen, and Richard Riehle), the short film Indian Gangster (2009), the drama Somewhere (2010, with David Jean Thomas), the short film Flawless (2011), and episodes of Futurestates (2010) and The Big Bang Theory (2015).
After six decades in Hollywood, De Souza is still acting and also works as writer, mainly articles for supplements published by the Times Group and the online magazine "The Label". In 2016, he was nominated for an ICG Publicist Media Award.