Only five days remain until the arrival of Doctor Who’s 10th series. And, just before the European premiere of episode one in London, showrunner Steven Moffat and new companion Pearl Mackie sat down with journalists for a chat about what will be Moffat’s last series and Mackie’s first.

In fact, for Mackie, having previously been primarily a theatre actress, it’s her first time on such a big set – and particularly one that utilises so many costumes and special effects.

“Every day is pretty surprising,” she admits. “I mean, meeting monsters wearing half prosthetics and half jogging bottomed attire at 7.30 in the morning is surprising.”

When she first began filming, Mackie was still appearing on stage in London’s West End as part of the cast of The Curious Incident of the Dog in the Night-Time. A car was sent to pick her up after the show and to drive her to Cardiff for her first day on set the following morning, when they hit a snag.

“The car broke down on Piccadilly [less than a mile from the theatre],” she recalls. “I got to Cardiff at about 3am and I was on set at 6am. Obviously I was more nervous than I’ve ever been in my entire life. I couldn’t even call anyone to talk to them about it because it’s all top secret.”

She was, however, given a warm welcome when she arrived, and was even sent flowers by her predecessor Jenna Coleman.

Mackie plays Bill Potts, a woman who works in the canteen of the fictional St Luke’s, Bristol University, who encounters the Doctor at the beginning of the first episode, The Pilot. As far as we know at the moment, she’s a pretty ordinary person – unlike Clara Oswald (Jenna Coleman), whose unordinariness played a major part in the past three series. In fact, both Moffat and Mackie emphasised Bill’s normality.

“It’s not been a life of tragedy,” Moffat chipped in. “It’s been a life of gentle disappointment. There are sadnesses but it’s just that her life hasn’t been going quite great. That’s it.”

When it comes to crafting a brand new character from scratch, Moffat says that he doesn’t have a checklist in mind or a plan to make them so deliberately different from the last.

“People aren’t opposites of each other,” he points out. “The kind of people who travel with the Doctor are going to have certain things in common. On the other hand, as you can see Bill is very different from Clara. You start kind of with the tone of voice. So I just started writing the very first scene that you see tonight, that’s the first thing I wrote of Bill just to find my way in.”

Most of the series was written after Mackie was cast, so, as executive producer Brian Minchin points out, it’s “quite organic”.

Even though this is Moffat’s last series on the show, he hasn’t been spending it running through a wishlist, saying, “Your wish fulfilment moments are more likely to come at the beginning.” Although he has done it for other people, such as allowing Peter Capaldi to face his much loved, rarely seen Mondasian Cybermen in the forthcoming series and David Tennant to battle some Zygons in the 50th anniversary episode.

Broadchurch’s Chris Chibnall will take over as showrunner from Moffat after the Christmas special. Which begs the question, what is really involved in handing over one of the world’s most beloved TV shows?

 Moffat replies, without missing a beat and to much laughter: “Have you seen that scene in Women in Love where the two men wrestle? A little bit like that.”

For those who aren’t familiar with Ken Russell’s 1969 adaptation of the DH Lawrence novel, that comment is even funnier than you may realise. Moffat is referring to a highly explicit, really rather long, and entirely unflinching scene in which Alan Bates and Oliver Reed completely (and we do mean completely) disrobe and wrestle each other in front of a roaring fire.

But, really, he says that it’s surprisingly straightforward, as Chibnall has already run a major, and incredibly popular, show before. They mostly just keep each other up to date on what’s going on. “He’s actually the most experienced showrunner who’s come on to Doctor Who.”

Most interesting is the cut and dry nature of the handover itself, which is fairly literal and pretty much exactly the same as the handover between Russell T Davies and Moffat in 2009. At the point where Capaldi finishes his regeneration and the new Doctor emerges, which it appears has already been filmed, Moffat is done.

“The best, and most hilarious, thing about it is, of course, is that I stopped several minutes before the end of my last episode and he takes over. Which is where I started,” he explains. “I started on Doctor Who at the moment David Tennant disappeared and Matt Smith appeared. I wrote that bit, which is a ridiculous day. On that day, I came in and there was Julie [Gardner, executive producer] and Russell sitting there watching the monitor. And then we bring Matt in and they handed over the headphones and said “goodbye, goodbye” – which is ridiculous! It was just insane.

“It’s quite easy to hand something over. It’s quite hard to get something handed to you. So I was doing the difficult part last time and the easy part this time which was, ‘here you go, here’s some Nurofen, off you pop’.”