Addition by subtraction: Actors (or hosts) who improved a show by leaving

George Dzundza originated the older-cop position on Law & Order as Detective Max Greevey, but grew increasingly frustrated when he realized that he was part of an ensemble show that would have little use for his character after the halfway mark of most episodes. He quit at the end of the first season, and was replaced by Paul Sorvino. Over the course of 31 episodes, Sorvino gave his character, Phil Cerreta, a degree of beefy warmth and intellectual cunning that Dzundza’s performance, which had seemed fine at the time, never came close to. But partway into the second season, Sorvino left to escape the rather hectic pace of a series shooting schedule. That cleared the way for Jerry Orbach to pick up the gauntlet as Detective Lennie Briscoe. Orbach gave a performance that made him the apotheosis of the senior-cop role and unofficial mascot of the NYPD. (Real cops loved the idea that this sharp-eyed cynic with a questionable past was one of their own.) On a show where heavy turnover among cast members had become the norm, Orbach played Briscoe for 12 years, right up until the actor’s death from prostate cancer.

The Vietnam War series China Beach was set at a seaside evacuation hospital and R&R facility, and featured an ensemble cast headed by Dana Delany as an Army nurse. The concept for the show was to depict the war as seen through the eyes of the American women who got closest to it. The character of the USO performer Laurette Barber (Chloe Webb, best known for her movie debut in Sid And Nancy) seems meant to provide viewers who had trouble identifying with career medical personnel or the prostitute played by Marg Helgenberger someone they could relate to. But the idea that women—or anyone—would see themselves in the USO singer is borderline insulting: She’s supposedly come to Vietnam partly because she thinks that doing tinselly covers of Motown hits for shell-shocked GIs will make her a star, and partly because a combat zone strikes her as a good place to meet guys. Webb, whose character did little but pull attention away from the real drama at the show’s center, left at the end of the first season and was replaced by Megan Gallagher, whose character—an ambitious war reporter—at least had a good reason for being there.

Marissa Cooper dies in a car accident at the end of The O.C.’s third season, but the character’s true cause of death was “evolution of show.” Mischa Barton’s poor little rich girl was a central figure in The O.C.’s early stages, when the show was positioned as a run-of-the-mill teen soap with agreeable taste in music. But as focus and fan enthusiasm shifted toward quirkier residents of Newport Beach—portrait-of-sarcastic-Barsuk Records-informed-angst Seth Cohen, Marissa’s deceptively cunning best friend Summer Roberts—Barton wound up swallowed by a run of hackneyed plots and increasingly “scandalous” love interests. In interviews following her departure from The O.C., Barton alluded to exhaustion with the show’s production schedule, though it’s just as likely that her character had exhausted her potential. In Barton’s absence, Autumn Reeser was bumped up to series-regular status; her insecure overachiever Taylor Townsend gave The O.C. greater opportunity to be the smartass meta-soap that was too often masked by Marissa’s weekly crises. It was a creative renaissance, but not a commercial one, and The O.C. followed Marissa into the dark after an abbreviated fourth season.

Archie Bunker may have outmatched Mary Richards in terms of ability to shepherd other characters into their own shows, but All In The Family never lost characters that were as crucial to its ensemble as Rhoda Morgenstern and Phyllis Lindstrom were to The Mary Tyler Moore Show. In a feat of tremendous TV fortitude, the most important female-driven comedy of the ’70s lost two of its bedrock actresses (Valerie Harper and Cloris Leachman) to spin-offs without stumbling. The key to Mary Tyler Moore’s continued success post-Rhoda and Phyllis were mid-series acquisitions Betty White and Georgia Engel. The energy and comic presence White and Engel brought to the show (White as vixenish TV host Sue Ann Nivens, Engel as sweet-but-naive Georgette Franklin) meshed with the brash personalities played by Harper and Leachman—but were also distinct enough to minimize the impact of Rhoda’s and Phyllis’ exits. White even managed to continue Mary Tyler Moore’s tradition of Emmy dominance, picking up the Outstanding Supporting Actress in a Comedy prize in 1975 and 1976—a category that was Harper and Leachman’s (and, in a one-time-only tie with All In The Family, Sally Struthers’) for the four prior ceremonies.

It would be unfair to blame Denise Crosby for the mediocre quality of Star Trek: The Next Generation’s first year, but her tenure as Tasha Yar is one of the many low points of the series’ debut season. Inconsistently written, Yar seemed grossly incompetent and undercut the show’s progressive vision of the future. The lack of attention paid to her character was Crosby’s reason for leaving the show, and after Tasha’s death, the producers shuffled around the cast and eventually retooled the series, giving Michael Dorn’s Worf and LeVar Burton’s Geordi larger, more interesting roles, both of which contributed some of Star Trek: TNG’s strongest ongoing plotlines.

While Grey’s Anatomy was initially built as an ensemble show centered on the hellish lives of a group of new doctors, as the show aged, it gradually took on more and more characters. Older actors came on as colleagues and love interests, and younger actors showed up as a newer, more naïve crop of interns. That’s why it wasn’t that big of a deal when Katherine Heigl got fed up with the behind-the-scenes drama and demanding schedule of the show and asked for the departure of her character, Izzie Stevens. Though Heigl seemed like one the more promising stars on Grey’s at the time, what with the success of films like Knocked Up and 27 Dresses, it turned out that the show was strong enough to withstand the loss of one of its key players—especially one whose character had taken such a turn for the annoying.