The beauty of TV (and occasionally its biggest pitfall) is that, for the most part, the producers, writers and directors are all essentially laying the tracks while the train they're on is already moving.
While the creators may have a general idea of where they want to take a series, those plans can change in an instant depending on what new writers bring to the table, the availability of actors and, of course, dreaded notes from network executives.
That level of spontaneity and the ability to make it up as you go along has been seen as a benefit though, and it's resulted in plenty of of TV shows that ascended from being good to all-time greats.
Plans change equally as quick when it comes to characters as well of course. So many times actors have been cast for a couple-episode stint only to be so impactful that the writers kept them around for years.
While that's impressive, what's even more shocking is the revelation that some of the best recurring characters on TV were only supposed to be around for a single episode and, in extreme cases, a single scene inside those single episodes.
8. Bobby Singer - Supernatural
By design, Supernatural wasn't intended to have many recurring cast members. Unlike most shows, for years the horror series only had two regulars in the form of leads Jensen Ackles and Jared Padalecki, with most of the characters they interacted with on their hunts being treated as one-and-done appearances.
That changed over time though, and the likes of Castiel - who was originally only supposed to appear in a small three-episode stint - ended up hanging around for years, even after dying.
In the same vein, Bobby Singer, Sam and Dean's daddy replacement, was only initially envisioned as a one-off character, born due to a scheduling conflict with another actor who was supposed to reprise her role from a former episode.
In the Season 1 finale, Sam and Dean's father has been kidnapped and they're in desperate need of help. They call up Bobby - an apparent long-time friend who hasn't been mentioned previously - to help them out.
The great work of actor Jim Beaver, and especially his paternal chemistry with the two leads, made him a hit with audiences, and the writers brought Bobby back for a few episodes in Season 2. From there he became one of the most popular characters to ever grace the show, and stuck around on and off (both dead and alive) until the show wrapped up in Season 15.