In the very first episode of "ER" — the landmark medical drama created by "Jurassic Park" novelist Michael Crichton — we meet John Carter, an ambitious but obviously overwhelmed third-year medical student played by Noah Wyle. Alongside other original cast members Anthony Edwards, George Clooney, Eriq La Salle, and Sherry Stringfield, just to name a few, Wyle's John helped shape the word of "ER," a gritty, realistic medical show set in Chicago's fictional County General Hospital. Wyle and Carter did leave "ER" full-time in the show's 11th season — though he made cameo appearances as the show drew to a close in its season 15 — but when John Carter finished out his run on the "ER," he became the show's longest-running character.

So why did Wyle leave the show that made him famous? In April 2005, CBS News reported that Wyle would leave the show as a regular but agreed to return for four...