The sixth series of the BBC police drama ended on Sunday night with many viewers disappointed when the corrupt officer working inside the force was finally revealed to be minor character DCI Ian Buckells, played by Nigel Boyle.
Mercurio told the Obsessed With... Line Of Duty podcast: "It was really about the heritage of the series, going all the way back to season one, where Dot Cottan says, 'Oh, can I just have a quick word with Tommy Hunter in the back of the van?' and Buckells goes, 'Yeah, alright'.
"OK, like, really, is that what you should be doing? Of course, not."
The writer also defended his decision to make supporting character Buckells the long-sought after 'H', and not the Police Chief Constable Osborne — played by Game Of Thrones star Owen Teale — who has been strongly hinted to be corrupt throughout the show's six series.
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Mercurio explained: "It was always really important to me that it was someone who had been in season one, someone who had been there the whole time.
"We did introduce some characters late, so Thurwell [James Nesbitt's character] came in late, we went back to Fairbank, who’d appeared in season three, we had the Chief Constable, who had obviously been there since season one.
"But then I don’t think he would have been a satisfying candidate because he had been absent for so long, it would feel like we’d just brought him back in to pin it on him."
The BBC crime drama — which began on BBC2 in 2012 — has followed the investigation of insiders in the police force working with criminal organisations over the past five series. Their long-running hunt for 'H' has always led viewers to believe the identity would be that of someone very senior within the police force and possibly even a major cast member within the AC-12 team.
While Boyle has played Buckells since the first series, he has always been a very minor role who slipped under the radar, and so his exposure as 'H' left many disappointed.
However, Mercurio has hinted there may be another series of the show.
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