Ben Linus was only supposed to be in a few episodes of Lost, but his character was so well received, that the writers made him a central character. Which proves that they really DID have a plan for the entire show all along…
On Boardwalk Empire, the character of Richard Harrow was only introduced in episode 7 of season 1, and only had a little screen time in the rest of that season. It wasn’t until after the first season had aired that the writers realized he was a fan favorite, and he became a series regular for seasons 2-4.
Still waiting for a contribution from baby Joe on Modern Family. Most of the time, he seems to be MIA–just a prop in the opening sequence. I don’t think he even attended Mitch and Cam’s wedding.
If minor season 1 characters that go on to take a major role count then Dean Pelton on Community was a big addition.
I didn’t like Tara, either. She was boring.
In “The Man from U.N.C.L.E” Illya Kuryakin (David McCallum) was supposed to be a minor character
but quickly rose to co-star status.
Chatsworth Osbourne Jr (Steve Franken) on “Dobie Gilles” was the snotty, rich brat so effectively the producers limited his appearances to keep him from taking over.
Dawn added not just a dynamic but an entire story arc and effectively transformed the overall narrative of the remainder of the series. Not to mention the way her advent was hinted at in dreams years before her actual appearance; has that ever been done in anything?
Adding Ice-T to Law & Order: SVU as Detective Odafin “Finn” Tutuol was a stroke of genius, and gave a new dimension to John Munch! I love the interplay between the two.