When the new series of Doctor Who needed an actor to play Charles Dickens, there was only ever one man in the frame.
Simon Callow has portrayed the Victorian literary giant on TV in two series of An Audience With Charles Dickens, re-creating his famous public readings, and has a wealth of knowledge about the author and his work.
That's not to say he would have said "yes" to Doctor Who if the script by Mark Gatiss, of The League Of Gentlemen fame, had not impressed him so much.
"When I heard that Dickens was going to be in Doctor Who, my heart rather sank," Simon admits.
"I get sent a lot of scripts which feature him as a kind of all-purpose Victorian literary character and really understand little, if anything, about him, his life or his books.
"But, as well as being brilliantly written, Mark's script was obviously the work of someone who knows exactly what Dickens is all about."
The story, set at Christmas in Cardiff in 1869, sees Dickens become involved with the Doctor and Rose as they encounter mysterious, gaseous creatures called the Gelth with sinister plans to populate our world.
When viewers first see Dickens, before a stage performance, he is tired and jaded, brooding on the mistakes he has made in his life. But his adventure with the Doctor opens his eyes to a whole new world and serves to reinvigorate his zest for life.
"Dickens was indeed very ill and unhappy at that time," says Simon.
"The script very cleverly connects his idealism, which ends up being restored by his experiences, with the Doctor's desire to save the world.
"Initially, he is shattered by the notion that this realm of the unknown, which he has always dismissed, actually exists, but he then embraces it. As the Doctor tells him, he's not wrong about everything, he just has more to learn."
An acclaimed actor, stage director and author, Simon confesses he was never a fan of Doctor Who: "I saw the very first episode in 1963, with William Hartnell as the Doctor, and decided it wasn't for me, so I missed the entire procession of Doctors that followed."
But that's not to say he wouldn't have minded a shot at playing him.
"When the BBC decided to bring Doctor Who back as a feature film a few years ago, one national newspaper ran a poll to ask its readers who should be the new Doctor, and I topped it," Simon recalls.
"Sadly, the producers failed to take note of this highly important statement of public opinion, so I never got my chance!"
The Doctor comes face-to-face with Charles Dickens in the new series as he battles against re-animated corpses and shimmering blue entities up to no good in Cardiff in 1869 - right up Mark Gatiss's street, in fact.
The League of Gentlemen star and massive Doctor Who fan says: "The original idea came from Russell T Davies, but it was ideal for me - a Victorian ghost story set at Christmas with dead coming back to life!
"I've always had this thing about possession. Alan Bennett once said that we all have only a few beans in the tin to rattle, and I do tend to keep coming back to the idea of things being possessed.
"They're always my favourite kind of stories and it really must scare me on some basic level, the concept of being occupied by other entities."
Mark says he's always loved Doctor Who storylines which involve travelling back in time as well as far into the future.
"Part of the fun of those historical adventures was seeing a Fifties police public call box standing on a Chinese plain in 2000 BC," he laughs.
Almost a year on from being asked to write for the new series, Mark still almost cannot believe that Doctor Who is returning.
"I'd ruled out in my own mind the possibility of it ever happening, so I've been pinching myself on set, seeing a beautiful new TARDIS, a brand new gold Dalek, and knowing this amazing new series is on its way.
"It's extraordinary, but I've always carried in my heart a version of how I thought it should be, and it's this one. It's a new show, not one addressing a dwindling bunch of Doctor Who diehards like me, but a modern show for a new audience.
"People who have never heard of Doctor Who should be able to sit down, watch and say 'I love it'.
"Being asked to write for the new series was the best present I've ever had. But having wanted the show to return for so long, it was also a bit daunting and I think we [the writers] all ran around like headless chickens for a while.
"But then you just have to get on with it and the hard work really starts, but it's always a joy because of the love and loyalty we have for the show."
Mark adds: "I think Chris makes a great Doctor, and they're both clearly men having fun. But that's what Russell said from the start - whatever he's up to, whatever danger he's in, the Doctor's having a good time."