And so, here we go again – it’s all change for the Time Lord. Doctor Who invented the sci-fi reboot back in 1966 when the first Doctor William Hartnell’s health began to suffer. But rarely has the show faced such a combination of changes – with Peter Capaldi, showrunner Steven Moffat and Michelle Gomez, who plays Missy (the female version of the Master), all signing off this year.

The first symptom of the changes is John Simm’s return as the earlier Master in this season’s penultimate episode, World Enough and Time. He told Moffat he’d love to return as long ago as his final appearance in 2010’s The End of Time, when David Tennant left. “It’s a role no father can turn down,” Simm explained.

His arrival this week begins an extended finale that will take us as far as a Christmas special, Capaldi and Moffat’s swansong. With the end in sight for Gomez, Capaldi and Moffat, we asked them about Doctors past, present and future…

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Peter Capaldi

I love this show, but I’ve never done anything where you turn up every day for ten months. I want to always be giving it my best and I don’t think if I stayed on I’d be able to do that. I can’t think of another way to say, “This could be the end of civilisation as we know it.”

With episodic television of any genre, the audience wants the same thing all the time – but the instinct that leads the actor is not about being in a groove.

What’s the Doctor Who audience like?

The conventions are a big carnival with a lot of really smart people having a laugh dressing up. The show is quite a benign friend so you get a happy response – people shouting from their cars, “Doctor Who, where’s your Tardis? What are you walking for?”

It still amazes me how huge the show is. I was recently in Minneapolis – where Prince is from. I was recognised so many times by so many people, I couldn’t quite believe it – not because of the character, who I know is world-famous, but because it was me.

What’s the hardest part of being the Doctor?

Doctor Who is a hugely challenging show to write and to act in. It has to turn on a dime from comedy to terror to tragedy. It’s a children’s show that developed into something more complex, a bit more adult-orientated, but we have a duty to play to the seven-year-old as well as the 42-year-old. Sometimes you have to be more comic than you’d normally be comfortable with, but it’s important.

How would you describe your Doctor?

The Doctor is deeply sad – I think he always has been. When you’re wise and you’ve lived a very long time, that’s how you’d be. Although you have to be careful with very human emotions and the Doctor because he’s an alien. It’s more straightforward to play the human elements, but then it might as well be a cop show.

What’s it like working with Michelle Gomez?

She has such range and energy – and she’s very beautiful.

She points out that she, you and Steven Moffat are all from Glasgow…

Well, Steven isn’t technically from Glasgow – he’s from Paisley. But he has a Glasgow sense of humour. He always finds what’s funny about you physically and puts it in the script. You suddenly read this description of yourself that is accurate but not always flattering. His presence is more pervasive than most people think.

Scripts come in that need a little work. He goes through them and sorts that out. This season people have compared scripts with his name and without his name and say the others are better. But they’ve all got his touch.

What can you say about your regeneration?

I can’t go into the details. I know what happens, but I don’t know how it happens. Certainly it’s not straightforward. It’s more complicated than recent ones. That’s one of the appeals of being in the show – it has death at the heart of it. He’s the only hero on TV who dies again and again.

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Michelle Gomez

As Missy, aka the Master, did you ever think you’d get to play the longest-serving villain in showbusiness?

No. And I nearly didn’t. I was offered an audition for Ms Delphox, the villain in Time Heist, but I wasn’t available. [The role went to Keeley Hawes in the 2014 episode.]

I thought that was my only chance to be on the new Doctor Who and I was gutted. So I was moved to write to Steven saying I was such a huge fan and if in the future if he ever needed someone for a razor-cheek-boned villainess then it’s me. I didn’t think any more about it until my agent called and said, “You’d better sit down.”

What’s the Doctor Who audience been like?

I’ve been to a few conventions and there are so many people giving it a bit of Missy cosplay. It’s a really moving experience to be part of that. I’ll be going to those for a long time to come.

What’s the hardest part of playing Missy?

I’ve been training for this my whole life. If you look at my career – from Irvine Welsh to Green Wing and Taming of the Shrew – there’s a little bit of Missy in all of it, from silly voices, to falling down, to some touching moments and the odd psychotic episode – sometimes all packed into just one speech. And in a corset.

How’d you describe your take on the Master?

I like to think of her as Mary Poppins’s evil twin. The loving part of the Master/Doctor relationship is just the fact that there’s a lot of history between them – they were incredible friends at one point, but the relationship just went sour. I mean, they’ve knocked around the universe together for hundreds of years.

She’s always talking about missing her friend. I think there’s a throwback to the relationship between Roger Delgado’s Master and Jon Pertwee’s Doctor [in the early 1970s]. Then you could see there was history between them, and a mutual appreciation of each other’s wit and intellect. Missy harks back to that.

Is it a coincidence that you, Peter and Steven Moffat are all Scottish?

It’s no coincidence – it’s a plot. Me, Peter and Steven are all basically from the same city. It’s because Gallifrey actually looks a lot like Glasgow. The Weegie sense of humour and attitude did give us almost a shorthand to communicate with – it’s a little bit of extra chemistry.

Which is the scariest monster?

When I was little, my brothers used to scare me by saying, “The Cybermen are coming.” Just that. I would freak out. Because that’s what’s so scary about the Cybermen – that they are coming.

The big reveal [during Missy’s first series in 2014] that the woman in charge of Heaven was really the Master, that I was in command of an army of Cybermen and I was surrounded by quite a large number of the metal men… That’s a day that will stay in my mind for a long time to come.

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Steven Moffat

What’s it like reaching your final series as showrunner?

Well, we’ll show up again at Christmas – apart from Michelle. I seem to have been reminiscing my way out of the door for about two years. It’s the longest departure in human history. I never intended to do it for ever and the workload is staggering.

I’m just at the end of my toughest ever year in which I’ll have done – in about a year – three Sherlocks and 14 Doctor Whos. That’s been shattering. Doctor Who has to move on and be different and shed me like a scale of a mighty dragon as it shambles off to another battle.

What’s the scariest monster you’ve created?

I think the Weeping Angels. We used to stay in a hotel in Dorset and next to it was a marvellous church with a chained-up graveyard and the words “unsafe structure” on the gates. I looked into the graveyard and could see a stone angel with its face in its hands as if lamenting.

I put the chained-up gates into the very first shot of Blink [a 2007 episode] and then I made up the Weeping Angel and I was very proud and excited. A few years later I said to my son Joshua, “Come and have a look at this. I’ll show you the original Weeping Angel.”

We went back to the graveyard and the angel wasn’t there – it was gone. So either I misremembered or they’ve moved it or there’s a real Weeping Angel in the world moving around when you’re not looking.

What’s the hardest thing about writing Doctor Who?

There are degrees of difficulty rather than degrees of ease. The Day of the Doctor [the 50th anniversary special in 2013] with all the pressure on the storyline and all the stars appearing in it was just a nightmare. I was never so happy with anything as when that became such an immense hit.

How would you describe your Doctors?

He is someone who’s running towards everything at once because he might miss it. He doesn’t understand why anyone would do the same thing every day or sit in the same room every day. He doesn’t understand why you would live a life in safety when you could be running from fires and explosions. He doesn’t understand why we volunteer to be dull – he needs to be out there and experiencing everything at once.

Along the way, of course, he helps people and people start to think of him as this great hero, but he doesn’t understand that – he’s just running past people and seeing that they need help, so he helps. Actors either have it or they don’t. The first time I saw Matt Smith – only the second person to audition for the role – you could instantly tell that he was Doctor Who. There was nothing clever about saying, “Well, obviously it’s him.”

How do you bow out?

I’ll strike a balance between not revealing spoilers and trying to get people to watch. I’m honour-bound as a writer not to give a damn about my departure. There’s no story about me leaving, the kids don’t know I exist – you can’t really get me away from the balcony when nobody knew I was in the building.

But it’s about giving Peter a grand, several-stage finale and trying to refresh the story that Doctor Who gets involved in with a tremendous crisis, during which he gets in some way mortally injured and has to turn into another actor.

Who would you cast as the next Doctor?

I think it should be me. That would be awesome. It’s about time they let a writer play the lead. I mean, I’ve been making up everything he says for years, so I can totally do it.

Doctor Who continues on BBC1 this Saturday 24th June at 6:45pm