Technically, you’re not even supposed to call John Hurt’s character the Doctor. The ‘War Doctor’, perhaps, at a stretch.

Doctor 8.5 was the Doctor Who incarnation we never knew existed until that jaw-dropping moment in 2013 when he was introduced. It messed with the history – but who cared, seeing as he was played by John Hurt?

Following Hurt’s death from pancreatic cancer at the age of 77, we look back in tribute of his fantastic cameo and examine exactly why the War Doctor – brief as his appearance may be – was so good.

In the first instance, the War Doctor existed partly because of Christopher Eccleston – who was reluctant to appear in the 50th anniversary special, despite a couple of encouraging meetings with Steven Moffat. Stuck for an idea, Moffat suggested ‘an incarnation of the Doctor none of us knew about – played by the most famous actor in the world’.

Doctor Who often does its best stuff when up against a creative wall, and this was no exception. John Hurt was top of Moffat’s list, and amazingly he said yes almost immediately. And the rest, as they say, is history, albeit one of those hidden histories that you never saw coming.

The first glimpse we got of Hurt occurred at the end of The Name Of The Doctor, when the Eleventh Doctor introduces him as ‘the one who broke the promise’, but we didn’t get to see him in action until November, when he strode across the wasteland of Gallifrey to bring about the end of the world (his world, not ours).

It would have been easy to turn the War Doctor into a grumpy, battle scarred veteran or a dark and sinister, almost unrecognisable figure – but he’s neither. Instead Hurt played him like a prospective father-in-law on a stag weekend, with a touch of Midsomer Murders. You could almost picture him poaching rabbits from a nearby forest before seeing something he shouldn’t and winding up face down in a patch of mud, skewered by one of his own bear traps.

But instead, he gets to hang about with two of his successors and out-perform them both. ‘He’s just moving his eyes’, says Matt Smith. ‘And I’m literally climbing off the walls.’ Tennant adds ‘I pull another face, and he’s imperceptibly raised an eyebrow, and you know he’s winning an Oscar right there’.

Nonetheless, many of Hurt’s best scenes are with Billie Piper (herself astonishingly good in this story) – who plays a sort of metaphysical Jiminy Cricket, invisible to everyone but the War Doctor himself. And it’s perhaps the best example of why this Doctor works so well – it gives us a chance to see how the Time Lord was ravaged by the tides of war without ever quite losing himself.

Because Hurt’s Doctor is weary, cynical and downcast (and anomalously bearded) but he’s still the Doctor. He carries a gun, but he’s later seen drinking tea from a china cup in the National Gallery. He’s removed, but instantly recognisable. His story has a beginning, a middle and an end – and it’s an end we finally get to see, even if the regeneration borrows directly from the very first, and is cut slightly short.

It’s also worth noting that Hurt took the role absolutely seriously – insisting to cast and crew on his last day that ‘This really meant something to me, to be the Doctor’ (Moffat’s paraphrasing). ‘He loves the fact that he’s Doctor Who,’ Moffat adds. ‘Only having to stay in Cardiff for three weeks, he gets to be Doctor Who.’

And personally, I think that’s what nails it. When an actor with Hurt’s stature and gravitas lends not only support but outright enthusiasm to a project like this, we feel vindicated. We feel that our obsession with Doctor Who actually means something in the grand scheme of things. This is no longer a silly, inconsequential sci-fi show: it’s something far greater.

Given the actor’s busy schedule, most fans thought 2013 was both the beginning and the end for Hurt’s involvement. But Day Of The Doctor wouldn’t be the last we saw (or at least heard) of this mysterious incarnation, with Hurt lending his voice to a series of Big Finish audio recordings chronicling the War Doctor’s earlier adventures.

It’s such a shame we’ll never get to see him on TV again, but we can revel, at least, in that brief, glorious appearance. The industry has lost one of its finest, most endearing and versatile actors today – a genuine national treasure, so many things to so many people – but to millions of Who fans, he will always be remembered as the War Doctor.