‘She was the only genius I ever met’: Wolf Hall director on making latest instalment without Hilary Mantel

The 10-year wait is almost over. Wolf Hall, the hit television series based on Hilary Mantel's trio of historical novels, will be back on the BBC next Sunday, leading viewers through the dangerous, maze-like political court of Henry VIII. This time, however, the path was darker and lonelier for director Peter Kosminsky , who had to proceed without his guide, the book's late author .

"It has been much harder, because Hilary is gone," he told the Observer ahead of the show's return.

"When she died, two years ago, I lost my collaborator and a good friend. Throughout making the first series, her attitude was always, 'Call me, night or day'. She was never protective or defensive, and I was looking forward to an even deeper relationship this time, since I had known the last book from its inception."

Kosminsky, who directed the acclaimed first series broadcast by the BBC in January 2015, grew close to the Booker prize-winning novelist as they worked to bring the content of the first books, Wolf Hall and Bring Up the Bodies, to television screens. As Mantel wrote the final book in the trilogy, she sent her friend 100-word instalments, eager for feedback.

"This was terrifying and bizarre," he recalls, "She was a double Booker prize-winner, so I had to be careful responding because I didn't want to bugger things up." Mantel's sudden death at the age of 70 then deprived him not only of "the person I respected almost more than anyone", but left him, Kosminsky says, with a heavy sense of responsibility and no opportunity to compare notes with "the best living writer in the English language".

"I don't say this lightly, but I think she was the only genius I have ever met, and I'm 68 now," Kosminsky said this weekend. "When in her company, as I was lucky enough to be a lot, it was clear she was operating on another level."

His own grief, the director knows, cannot compare to that of her family, nor of Gerald, her husband of 50 years, but Kosminsky's fear of letting the author down proved difficult to handle, he admits: "Thankfully, we were exchanging detailed emails while she was writing the last book, and then afterwards, as the adaptation was coming together. But I was expecting lots more as we went into production."

The final instalment, scripted again by Peter Straughan, follows arch-strategist Thomas Cromwell, played once more by Mark Rylance, as he struggles to maintain his powerful place at the side of Damian Lewis's increasingly mercurial king. The missing figure of Cromwell's dead mentor, Cardinal Wolsey, repeatedly appears to him in a corner of his study to advise him.

Kosminsky admits that he needed the same kind of reassurance from Mantel, although there was no such comforting apparition on set: "This time, we could not ask her if there was anything we should change."

The last book may be titled The Mirror and the Light, but its narrative steers readers inexorably towards the pitchest black of conclusions. So turning the story into a six-part drama became a test of nerve. not just for Kosminsky but for his leading actor. "The story ends in the darkest way, and my job was to go on that journey with my actor. Mark gives a remarkable performance. He is a one-off who goes into it body and soul, and it was hard for him.

"He is playing a man who started life as a boy in Putney being kicked daily by his father and who then enters into an abusive power relationship with the king. In Hilary's version, it becomes a kind of love affair, or at least a co-dependency, from which Cromwell cannot escape."

Mantel explained to him, Kosminsky remembers, that "the mirror" of the title stood for memory, while "the light" was Henry: "She was very clear to me that this was a more internal conclusion, a story that had to do with 'reflection' in that other sense, that is with looking back at the past."

A key concern was how to pick up the screen narrative after such a long break. "The story continues unbroken from the first series, and yet everyone was a decade older, including the audience. Then it struck me that the three novels cover the 10-year period that Cromwell served the king until 1540. So there's a jump at the beginning of this series, and viewers see the ageing of the characters. Then, of course, because it is television, we can use flashbacks."

Kosminsky is known for political dramas and he first worked with Rylance on the film he wrote about the death of Dr David Kelly , the government weapons inspector.

But Mantel, he said, was always clear to him that these books were "not an allegory" for modern politics. "I do think, though, that they are a kind of antidote to simplistic forms. After all, Hilary has taken one of the most important characters in history and reappraised him. She had the bravery and chutzpah to take a man universally considered an irredeemable villain and say, 'Hang on, this guy has his own story'."

The final episodes of Wolf Hall: The Mirror and the Light, will now go out on consecutive Sundays up until Christmas, and then Kosminsky returns to another bleak project. For seven years, he has been preparing a serialised drama about the Grenfell fire disaster: "Although I often deal with dark subjects, I do it primarily because I'm asking people to look at things again, using this incredibly powerful medium. It is a joy, and I am very lucky to do it."