When afterthoughts become a show's raison d'être. The Fonz is the classic example of this.
From Mental Floss.
ALEX P. KEATON - FAMILY TIES
At the urging of a casting director, show creator Gary David Goldberg gave a young Canadian actor named Michael J. Fox a second screen test, and reluctantly hired him. Much to everyone’s surprise, Michael J. Fox had an on-screen charisma that quickly made him an audience favorite; he could deliver the most absurd and extreme remarks and garner a laugh instead of a groan as long as he flashed that adorable smile. Meredith Baxter-Birney was just a bit miffed, because she was told that the parents would be the focus of the series. During the taping of the episode where Alex lost his virginity, the audience’s laughter went on so long that the show ran 12 minutes overtime. Goldberg was standing backstage with Baxter-Birney at the time and said to her, “If you want to leave the show, I’ll understand.”
CHRISSY SNOW - THREE'S COMPANY
Even though John Ritter was the acknowledged star of the show, it was Suzanne Somers who got her picture on all the magazine covers and had her own mega-selling poster. As soon as Somers landed the role of Chrissy, she contacted powerhouse manager Jay Bernstein and begged him to take her on as a client. She wanted to be “bigger than Farrah,” and Bernstein, impressed by her passion, agreed to manage her. Thanks to his savvy promotion, soon every episode of Three’s Company, no matter what the plot, focused heavily on Chrissy prancing around in tight T-shirts and short-shorts.
DARYL DIXON - THE WALKING DEAD
Norman Reedus originally read for the role of Merle Dixon when AMC’s zombie show was being cast, but that part was given to Michael Rooker. Nevertheless, producers liked something about Reedus, so they had the writers give Merle a younger brother named Daryl. The redneck bow-hunter was intended to be just another member of the ensemble that rounded out the main cast. But Norman took what could’ve been a one-note character and, with just a few lines of dialog per episode, made him intriguingly complex instead. By season three, Daryl (a character that didn’t exist in the WD graphic novels the TV show is based on) had become Rick’s second-in-command and rabid fans were frequently spotted wearing T-shirts warning “If Daryl Dies, We Riot.”
BEN LINUS - LOST
Michael Emerson was invited to make a guest appearance on Lost based on the strength of his Emmy-winning portrayal of a serial killer on The Practice. That initial appearance in the episode “One of Them” led producers to invite him back for three more episodes, still billed as a “guest star.” His morally ambiguous Benjamin Linus (originally known as Henry Gale) struck a chord with viewers, who loved to hate him, and as of season three, Emerson was offered a contract and became a series regular as well as the leader of the Others.
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