TV writers may have a vision for their brilliant new show, but there's one thing they can never predict: a random background character who suddenly becomes a fan favourite and now they have to change the entire series if they want to keep it on air, dammit.

These are the characters that helped make the show they were in, despite the fact they were only meant as a minor addition for an episode or two, if that. And we salute them for it:

1. Gunther (Friends)

Actor James Michael Tyler was famously cast as Central Perk boss Gunther purely because he was the only extra on set who could work a coffee machine on the first day of filming. Just think: if some other random dude put his hand up first, we would never have lovely Gunther and his unrequited love for Rachel.

Technically, Gunther appeared in the most episodes outside the six friends, appearing in 158 in total (though, a lot of these were background unspoken appearances, but still). He didn't have his first line until his 33rd episode, saying: "Yeah."

2. Summer Roberts (The OC)

Fox

Summer was only intended as a very minor supporting character at first. Her role was meant purely as another vacuous Orange County bimbo and a brief object of lust for Seth.

But, thanks to Rachel Bilson's performance and chemistry with the cast, creator Josh Schwartz gave her more scenes, saying: "She was adorable and funny and smart, which was a really fresh way to go with that character. And the more we gave her to do in the first couple of episodes, the more she kept nailing."

Summer went on to become the show's lead female character following the death of Marissa Cooper in season 3, and arguably the most popular character of all.

3. Randy Marsh (South Park)

In the early days of South Park, Randy was merely a background character who didn't say much and it wasn't even established that he was Stan's dad.

However, as creators Trey Parker and Matt Stone got older and started to identify more with the adults than the kids, Randy slowly became elevated to one of the main characters. And now, he's easily the most prominent adult in the show, and one of the funniest by far. From "cock magic", to secretly being Lorde to fighting rival dads, he's everywhere. And to think that he's still technically a geologist.

The same also applies to sweet little Butters, who was also a mute minor character who eventually became one of the most important and best loved characters in the show.

4. Aaron Pierce (24)

Fox

Glenn Morshower was essentially just an extra playing an unnamed Secret Service agent protecting the President in 24's first season. In the second, he had a couple of scenes with dialogue, but nothing huge. But fans and producers clearly liked his face enough so kept bringing him back.

By season 5, Aaron had become a kickass bodyguard, helped Jack Bauer take down an evil President, almost fell in love with the First Lady and stopped a conspiracy involving another President's irritating daughter. Not bad for an extra.

Aaron is the only character other than Jack to appear in and survive the first seven seasons of the show (he's sadly not returned since but we still hope).

5. Cigarette Smoking Man (The X-Files)

Back at the start, actor William B Davis's mysterious character was written as an extra for the pilot episode, and he returned for a couple of small cameo appearances in the first season. Clearly, he smoked that cigarette bloody brilliantly, because he didn't go away. Particularly impressive as he hadn't smoked one for 20 years at the time.

Eventually, he became the main baddie of the entire series, and was Fox Mulder's arch enemy and a key member of the government conspiracy known as the Syndicate.

And that wasn't all. After years of being presumed dead, he came back again for the 2016 revival, releasing chemicals that would kill countless people before ending on a cliffhanger. What a bastard!

6. The Janitor (Scrubs)

Neil Flynn's grumpy custodian may have appeared in every episode of the first season, but he wasn't meant to be such a prominent character at the beginning.

Creator Bill Lawrence has said that had Scrubs been axed really early on, the character would have been revealed to be in JD's mind all along, hence why he's the only one who interacts with him at the start.

But, everyone on set and at home loved the Janitor so much, he soon became a main cast member and one of the most hilarious stars of the show. He FINALLY revealed his real name – Glenn Matthews – in the finale. Why did it take so long? Nobody asked.

7. Ben Linus (Lost)

ABC

Perhaps proving that the writers made it up as they went along, Michael Emerson was only meant to appear in three episodes of the second season, but they liked him so much they added a further five.

By season three, they decided to make him the show's primary baddie, even though Emerson had no idea about his character's backstory for yonks (he wasn't the only one).

By the end of the show, Ben became one of the best and most loved characters (despite killing a bunch of folks), and even managed to survive the end and took control of the island as Hurley's right-hand man. We still desperately want to see that spinoff.

8. Castiel (Supernatural)

Can you imagine Supernatural without Castiel now? Not only did he not even appear until the fourth season, but he was only meant to have a six-episode arc.

But, by the end of that season, Misha Collins's angel had quickly become a huge fan favourite, and he was saved from death and became as important a star as main protagonists Sam and Dean Winchester from season five onwards.

So popular in fact, he came second in Digital Spy's poll to find the best TV character of the 21st century!

9. Steve Urkel (Family Matters)

Never before or since has a character literally stolen the show off its actual main cast like Urkel.

Jaleel White's geeky kid was meant to be a one-off character, but he quickly not only became the show's most popular star, but eventually its main protagonist.

Despite the fact that he wasn't even in the Winslow family, the show was all about Urkel after fans couldn't get enough of his thick eyeglasses, suspenders, cardigan sweaters and high-pitched voice.

See also: JJ in Good Times

10. The Fonz (Happy Days)

Much like with Urkel, Fonzie proved what would happen if you had too much of a good thing: jumping the shark.

But, long before that, Henry Winkler was cast at the last minute as the leather jacket-wearing hero, as original star Micky Dolenz from The Monkees was too tall.

Starting life as a fringe character, it didn't take long for him to become the main focus of the entire series. It got to the point that at one stage, ABC wanted to change the title to Fonzie's Happy Days.

11. Tommy Oliver (Power Rangers)

Power Rangers legend Tommy was only supposed to appear in the first season as a small character, but fans loved him so much that he eventually became the main star, and kept coming back for many versions of the show.

Originally the Green Ranger and later the super cool White Ranger, he went on to appear in Alien Rangers, Zeo, Turbo and Dino Thunder. At this point, he was revered as a legendary Power Ranger veteran by the latest recruits.

He also featured in Wild Force, SPD and Super Megaforce (we're not making these names up, by the way), and actor Jason David Frank has reprised the role every single time. Here's hoping for a cameo in the upcoming film, eh?

12. Ronnie Gardocki (The Shield)

FX

Despite being one of Vic Mackey's Strike Team of four, Ronnie only appeared in a few episodes of the first season, and actor David Rees Snell wasn't even credited for the pilot. He was actually just hired as an extra.

As the show progressed, Ronnie became a fan favourite and a more prominent member of the team, showing his loyalty to Vic and the crew more than any of the others. By the end, he was arguably the most kick-ass MF of the lot and we can't imagine the show without him.

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