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Sharing a shot of himself in character as Detective Inspector Steve Arnott, Compston wrote on Saturday (10 April):
Well, that sounds bloody exciting doesn't it?
Fans say they've been left 'terrified' by the post and what it might mean for AC12's DI Arnott.
One person wrote: "Well now I'm terrified, thanks Martin."
Another said: "This is my first time watching live do you know how terrified I am."
A third posted: "Nervous, excited, praying you are going nowhere. Sneaky feeling you might be suspended after drugs test, leaving AC12 in a much weaker position.
"Also, Ted could be suspended, AC12 are getting too close. Cracking series."
Speaking before the new season kicked off, Adrian Dunbar, who plays Ted Hastings, promised viewers plenty more 'twists and turns'.
In an interview with the Sun, the 62-year-old was asked if he knew who H was, to which he replied: "That is a very difficult question to answer. Jed does not let us know, does he? Who knows?
"The twists and turns of Jed's mind... anybody could be. In the latest trailer, the finger is pointing in all different directions that you would not have imagined.
"So the clues are there as to who people are. Things will be revealed in this series. I have never been in an ongoing series before and after series two I realised the writer has absolute control of your life.
"If Jed wants to, he could just throw me out of a window. So consequently we are all really nice to Jed. He is our best mate."
Bigging up his 'best mate' he added: "Jed has consistently upped his game. Every time we get the new series we think, 'Wow, he is taking the standard up'.
"If it was in America, after series two it would have been handed over to a team of writers and we would have done 13 episodes a year and very quickly we would have run out of steam.
"But because we have had one writer who has decided to stay with it because he loves it and it is his baby, he has kept the standard up quite remarkably over the years."
You can watch the fourth episode of season six of Line of Duty on Sunday (11 April) on BBC One at 9pm.
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