Bent coppers, heart-pounding interrogation scenes and thrilling twists have turned Line of Duty into the biggest drama series in Britain.

Heading into its sixth season on the BBC, the show’s passionate fanbase continues to grow and with Boardwalk Empire and Trainspotting star Kelly Macdonald joining as the new series' guest star, we could be in store for the best episodes yet.

Since launching in 2012 on BBC Two, Line of Duty has become a TV gamechanger, exploring the crime drama genre in ways we’ve never seen before, diving into the nitty gritty of real police work and exposing the dark world of police corruption.

The story of Line of Duty from niche BBC Two drama launched with little fanfare to monster BBC One hit is an unusual journey and here are some more surprising facts and bits of trivia you may not know about the series.

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Line of Duty season 6: Secrets from the Set

1. BBC One turned the show down

Lennie James in Line of Duty BBC One

It’s unfathomable to believe now, but BBC One initially snubbed Jed Mercurio’s show.

Simon Heath, CEO of Line of Duty producers World Productions, admits that when BBC One turned its nose up, the team started to believe that the show might never get off the ground.

There was a nine-month wait before BBC Two finally came in and the show was given the go-ahead. Good work fellas.

2. What happened to Tommy Hunter and Lindsay Denton was never planned

In an interview with Script Angel about his writing process, Jed Mercurio revealed that a lot of the plot twists and returning characters are not planned from the start.

Because the series is only commissioned one series at a time, the writer begins each series afresh rather than having grand multi-series plans for each character. Which is actually quite staggering when you consider the tightly plotted details and moving chess pieces that take place across the series.

“The Caddy and Tommy Hunter feature in series one but it wasn’t until we were commissioned for series two that I went back,” said Mercurio.

“We’d established an embedded corrupt officer and we felt that it would be great to use that more. The same process applied to series three, I felt a need to reinvest in that story and the same applies to Lindsey Denton’s story. When we made and delivered series two we all felt that it had a closed ending – Lindsey Denton’s going to prison for life.”

3. Martin Compston wasn’t the only person to audition for Steve Arnott

Martin Compston as Steve Arnott in Line of Duty BBC

In an interview with GQ magazine, Compston revealed that he was persuaded to audition for the show by Craig Parkinson – who would go on to play Matthew ‘Dot’ Cottan.

He also said that Parkinson also initially tried out for the role of Arnott – who had the name Steve Andrews in the original scripts.

4. The Ted Hastings catchphrases come from Adrian Dunbar himself

Hastings wasn’t originally planned as a major character, but Dunbar’s performance in season 1 forced Mercurio’s hand into making him a series lead.

He also has a strong hand in the colloquialisms – or should that be Ted-isms? – that have become such a popular part of the show.

“The ‘mother of God’ stuff is something that my dad used to say all the time,” Dunbar told GQ.

“It fits the character, you know. ‘Sucking diesel’s’ mine. It’s something I used to say all the time.

“Jed would ask me what about some really locked-in Belfast phrases, which ones, and ‘floating up the Lagan on a bubble’ is a big Belfast one. So we thought we had to get in somewhere.

5. A 21st century Bodie and Doyle

In a little nod to the 70s British crime drama classic The Professionals, viewers might have spotted Kate and Steve using the radio calls 3-7 and 4-5.

These were the same call signs used by Martin Shaw and Lewis Collins’ Bodie and Doyle in The Professionals.

6.   Jessica Raine is a real pool shark

Jessica Raine in Line of Duty BBC

The scenes in season 2 where DC Georgia Trotman showed off some secret skills around the pool tables were added by Mercurio when he discovered that actress Jessica Raine was a dab hand with a cue.

“She made every single pot on every single take,” said Mercurio.

7. Where’s Jed?

The show’s creator and writer has made a cameo appearance in the show, but you won’t be able to spot him no matter how hard you look.

Writing on Twitter about the first series, Mercurio said: “My only cameo appearance in #LineofDuty was invisible. I'm driving the grey Golf in front of Lennie James and Gina McKee. No one on set knew the route we'd recce'd so Lennie followed me through the streets of Birmingham while they filmed the scene in the car behind.”

8. The mystery of Tony Gates

During a Tweet-along of season 1, Mercurio revealed that they were forced to change the name of Lennie James’ character to Tony Gates after they had started filming.

That is why you’ll spot some clever editing in early episodes where the camera cuts away from Kate Fleming and Steve Arnott when they say the words ‘Tony Gates’. They got McClure and Compston to redo their lines in post-production using the new name.

Watch Line of Duty season 1-5 on BBC iPlayer.

Watch season 6 from Sunday, March 21 on BBC One.

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