Cassidy displayed some acting ambition by landing the lead in the TV-movie docudrama "Like Normal People" (ABC, 1979). He played opposite Linda Purl as half of a mentally handicapped couple who defy authority with their desire to marry. After the Hardy boys had closed their casebook, Cassidy returned to primetime as the star of the series version of the 1979 comedy-drama feature "Breaking Away" (ABC, 1980-81). Tiring of bubble gum, he tried to establish a more "serious" rock image with his 1980 album "Wasp". Produced by Todd Rundgren, Cassidy performed covers of songs by such heavyweights as David Bowie, Ian Hunter, Peter Townsend (of the Who) and David Byrne (of Talking Heads) but the public was unimpressed. Retreating to the hearth, Cassidy left recording and performing for several years to educate himself and spend time with his wife and two children.

Cassidy returned to acting the old-fashioned way--on the stage, appearing in regional theater. He popped up on TV fleetingly on the "American Playhouse" production of "Breakfast with Les and Bess" (PBS, 1985) supporting vets Dick Van Dyke and Cloris Leachman. Cassidy made a little-seen feature debut in an American Film Institute production entitled "Born of Water" but made a bigger splash in his London stage debut in "Bus Stop". He bowed on Broadway in 1993 alongside David Cassidy in "Blood Brothers". While Cassidy never totally forsook TV acting, making a few guest shots on the likes of "Matlock" and "Murder She Wrote" and telefilm appearances ("Roots: The Gift", ABC 1988), his interests in the medium were more directed behind the camera.

Cassidy took part in developing a mystery vehicle for Edward Woodward entitled "Over My Dead Body" (CBS, 1990-91). He landed a "consulting producer" credit on the pilot but the show was canceled before the episode he had scripted was produced. Nonetheless the teleplay provided him with a handy calling card. Demonstrating an interest in the macabre, Cassidy co-produced and scripted "Strays" (USA, 1991), a silly made-for-cable entry about a terrifying tabby and his crew.

After scripting another minor TV- movie (the 1994 syndicated "Midnight Run For Your Life" derived from the 1988 feature "Midnight Run"), Cassidy surprised the industry by reemerging as the creator, supervising producer and major writer of the highly touted "American Gothic" (CBS, 1995). A creepy hour-long merging of "Twin Peaks" and "The Andy Griffith Show", his pilot garnered critical kudos for its smart writing and acting and controversy for its unsettling violence, but despite the accolades failed to connect with a broad audience. Nevertheless Cassidy's new career as a series writer-creator-producer took off amd he quickly had other quirky series hitting the airwaves, again to critical accliam but low ratings: Fox's "Roar" (1996) was an adventure series follwing tribes of Irish warriors battling Roman invaders; NBC's "Players" (1997) following a group of young ex-cons as they work together to fight crime for the FBI; "Cover Me: Based on the True Life of an FBI Family" (2000), about a family in deep cover; CBS's "The Agency" (2001-2003) about the inner workings of the CIA; and The WB's "The Mountain" (2004) starring Oliver Hudson as a black sheep son who must deal with his estranged family when he inherits his grandfather's ski resort. Cassidy did rack up a modest hit with CBS's "Cold Case" (2003- ), another crime procedural from executive producer Jerry Bruckheimer focusing on a detective (Kathryn Morris) investigating unsolved homicides, but he left the series after the first season in favor of The WB's "Tarzan" (2003), which teamed a modern day version of the Ape Man with NYPD detective Jane and lastet only eight episodes. Cassiday rebounded strongly as the showrunner and executive producer of the ABC sci-fi series "Invasion" (2005 - ), which set the ongoing story of an alien invasion at a ground-level, human perspective and, nestled comfortably in the timeslot behind ratings titan "Lost," became a hit with viewers.