Colin Firth is entering the sequel phase of his life. He didn't plan it or even see it coming, but nonetheless, here it is.

After two wildly successful follow-ups to Bridget Jones's Diary, he is currently filming a sequel to Mamma Mia! and there is now the return to the role of Harry Hart, the suited spy he plays in Kingsman: Secret Service.

Being shot in the head by Samuel L. Jackson's eco-terrorist in the 2014 outing has not stopped him making a return in the highly-anticipated sequel, Kingsman: The Golden Circle, in which Harry must find his feet again in the world of espionage after sustaining a horrific head injury.

While fans feared it was the last they had seen of the suave secret agent, Firth always believed director and co-writer Matthew Vaughn would find a way to bring him back from the dead.

"I think I always believed there must be a way I come back," he says. "I thought 'you're not going to do it without me', which is probably what every Game Of Thrones and Walking Dead actor thinks before the axe falls. But no, there was no plan to bring me back during the first one. I just double checked that with Matthew this morning, and he said no, you were definitely dead, but I kept thinking: 'Well, you know, if you didn't see my body too closely and you weren't at the funeral - there was no follow up - then I'm sure there is some room to manoeuvre'.

"But I don't think Matthew cared too much about that. We all get on very well and I think he just thought, 'well this is the team that works' and these are the relationships he wants to work with again, so we must find a way."

The sequel sees Harry reunited with his protege Eggsy, played by Taron Egerton, and the introduction of some new faces, including Channing Tatum, Jeff Bridges, Halle Berry and Julianne Moore.

Firth has been a working actor since the 1980s but has rarely reprised roles, until the huge success of the adaptation of Bridget Jones's Diary called for a second film, and then, last year, a third.

"I'm beginning to (return to roles)," he muses. "I have gone into the sequel phase of my life now, which has suddenly come upon me. You suddenly realise that you're being haunted by your past. I suppose I like to look at it, if we're doing sequels, somebody must have enjoyed the first one."

It's not something he has sought out, he says. Playing Mark Darcy in Bridget Jones, which was a deliberate reference to his portrayal of Mr Darcy in the BBC's beloved adaptation of Pride and Prejudice, was quite enough.

"It's not something I look for. It's enough that you play characters that resemble other characters that you play, without going and putting on the old pair of swimming trunks again. But this one doesn't feel quite like that, because they're so close together and we've not lost contact with each other. It almost feels like we're on one long project rather than making sequels. I think this again, talking about a phase of life, this is my sort of Kingsman period."

It is still shocking to him that now, seated serenely dressed all in black in a London hotel room just days after his 57th birthday, his Kingsman period also means his action star period.

"I definitely would not have seen that coming. I thought I was condemned to point my suit at people for the rest of my life and the idea that anyone would ask me to exert myself physically, I thought if that was ever going to happen it would have been 30 years ago."

It just so happens that 30 years ago he was starring in Another Country, based on the life of Cambridge spy Guy Burgess, and pictures of his twenty-something self, floppy hair and chiselled jaw and all, have recently resurfaced on the internet. This is news to Firth, who looks aghast. "Really?" he asks. "God. I'm not going to look at it.

"It's odd, as you age you realise that you now look different than you did two years ago and then three years ago and then four years ago.At first, you know, of course we all change a little bit, but then you reach the point when people are saying, 'can you believe this is the same person?' So probably I am at that threshold now."

Few would argue that the years have diminished his charm, but there have perhaps been some surprises along the way. One of the more unexpected turns pops up in the Kingsman sequel, when he shares the screen and a fighting scene with Sir Elton John.

"Now that I didn't foresee either," he says. "It was a wonderful, wonderful experience. Often when you're on a set and you're shooting a scene and you hope the effect of the scene is going to be fun and extraordinary, but the actual work that you're doing seems to be rather technical and drawn out. That wasn't. That actually was a pinch-yourself moment, just in the doing of it because I had him there."

He is anxious not to reveal any plot points that would ruin the story for viewers but he adds: "We even had music playing on the set and the stuff that didn't make the final cut, which I really do hope people see in some version somewhere in the future because this film would have been six hours if everything had gone in ... but no question about it, that was the highlight not only of the film, but of all time for me."

Kingsman: The Golden Circle is released in UK cinemas on September 20.