When George Clooney and Julianna Margulies scrubbed out, Goran Visnjic and Maura Tierney scrubbed in!

Luka and Abby’s on-again/off-again relationship would prove as central to latter-day ER as Doug and Carol’s relationship was in those first few seasons, and their respective backstories — Luka’s first wife and two small children perished in war-torn Croatia; Abby struggled with sobriety after growing up with a mentally unstable mom — went a long way toward helping viewers warm to them individually.

https://tvline.com/lists/best-tv-characters-late-cast-additions-seinfeld-buffy-the-office/best-tv-characters-late-cast-additions-luka-abby-er/