The actor, who grew up in Dunoon, was, after all, once part of The Ken Campbell Roadshow, a comedy act in which he would regularly set his head on fire and push 6in nails up his nose and ferrets down his trousers.

Even as a youngster, little Percy James Patrick Kent-Smith, as he was then, was so full of derring-do and daftness he would take little rowing boats out to sea and deliberately bump into giant ships.

"Yes, I did," he says. "But I'm not so active these days. And I have certainly abused my knees a bit over the years. It is not surprising they are giving me a bit of ache."

McCoy's 68-year-old cartilage will not stop him taking to The Tron stage, however, to appear in Plume, the "beautifully written story of one man's grief and anger following the death of his son in a terrorist attack on an aeroplane".

This role seems rather dark, however, for the impish actor who became the seventh Dr Who between 1987 and 1989.

"Yes, and that's why I wanted to do it," he says. "But, then again, I have often done comedy in which I play darker characters, such as my appearance in Still Game, where I appeared as the sad little man. And I once played a lunatic in Rab C Nesbitt."

JC Marshall's new play is not based around the 1988 Lockerbie tragedy.

McCoy says: "No, in a way the writer wants to keep away from comparisons and wants to focus on his own characters. And this is not a political story."

"I have two sons so I suppose I will think of how I would feel if something happened to them. But, to be honest, I have not gone there in my head. It will be interesting to see how the role affects me.

"And it is a concern. I once played a character in a film, based on true life, who kidnapped a girl and stuck her in the wheelie bin. After I had come off set I had to go and try and make people laugh.

"I guess it was a way of dealing with the darkness." He breaks into a smile as he adds: "Perhaps after this show I will rent a pub and do stand-up."

McCoy has seen darkness in his own life and he coped.

His dad, a submariner, died before he was born and his mother died at an early age, having suffered from mental illness.

"If you are not a orphan you expect to be loved; you don't have to do anything extra," he says.

"But when parents are not around perhaps you go looking for that little bit of added attention."

McCoy was looked after by assorted aunties and grannies. "I was cursed in one way, but in another I was blessed with the means to deal with it," he says.

"I had sunshine in my personality."

The producers of the Hobbit clearly recognised that – McCoy will star in the two upcoming Hobbit films.

Yet, he reveals filming on the biggest two movies made this year (a $500million production starring the likes of Sherlock's Martin Freeman and Benedict Cumberbatch) was rearranged – thanks to his Tron commitments.

He says: "I filmed eight weeks in New Zealand at the beginning of this year and they wanted me to go back again for three days.

"But those three days coincided with appearing in this play. So I asked The Hobbit people if they could move the dates.

"Now, my agent told me there was no way the Hobbit would wait for me.

"But I had a feeling things could be sorted. You see Peter Jackson (producer and director) and his wife are such smashing people.

"I have had a great adventure. And it is a lovely part.

"In the book, Radagast is a character who is talked about, he is slightly off stage, but in the film he is certainly on stage and they have given him some really nice things to do."

McCoy seems to take the recent film success in his stride.

He adds: "For me, a job is a job, whether it is this play or the Hobbit films. I can get equally involved in either.

"And if I didn't take this play so seriously I would not have asked for time off from filming.

"I think I am lucky to be doing this play as well. I have always felt this way.

"As soon as filming was over for Dr Who, I went back to the National Theatre to do the Pied Piper.

"People were saying I should cash in and at least do commercial theatre, but for me it was about the work."

The actor was almost a Hobbit 10 years ago. "I screen-tested for the role of Bilbo Baggins. And I was in the frame for a long time," he says.

"Then it came down to me and 'someone else'. And that someone else got it. (Ian Holm)

"Yes, I was a bit disappointed. I was younger and perhaps such as job would have changed my life.

"But I do not place work at different levels. Performing at a working men's club in Hull or a theatre in Glasgow is just as important to me as a Hobbit film."

l Plume, Tron Theatre, March 1-17, also stars Gemma McElhinney and Finn Den Hertog.

FORMER Dr Who star Sylvester McCoy will soon be better known as a wizard in The Hobbit. But as BRIAN BEACOM discovered, he was able to persuade Peter Jackson, the director of the $500million movie which was filmed in New Zealand, to delay filming his scenes so he could appear on stage in Glasgow this week