I got to thinking about this while going through the thread on actors/characters who vanished without trace from a TV series. This is the opposite. Let's talk about actors who appeared in a series initially as a one-shot guest appearance but were so memorable, they were eventually added as a key character in a series. The Buffy/Angel franchises had a few--most notable James Marsters as Spike. The original plan was for him to be killed after in his third or fourth appearance of the Drusilla arc in the second season of Buffy. But the character was so good, the arc was rewritten to where he survives and remains as a key guest star throughout the season. He returned for a one-shot appearance in the third season of Buffy, was really good again and eventually emerged as a series regular in the fourth season. He stayed through the rest of Buffy's run, was supposed to die in the season finale. But still, Spike again returned from the dead to appear in the final season of Angel on the network's insistence. Other Buffy/Angel guest stars-turned-key characters were Alexis Denisoff as Wesley, Emma Caulfield as Anya, Julie Benz as Darla. Possibly one or two others. Juliana Margulies' Nurse Hathaway committed suicide in the pilot of ER. But she was miraculously resurrected after the series got picked up and was made a regular cast member. Hill Street Blues had two officers who were resurrected from the dead to join the cast -- Renko (Charles Haid) and Joe Coffey (Ed Marinaro). Those are just off the top of my head. Who are some others?