"What if the Doctor were given a companion who was up to no good?" So went one of the questions at a script conference with Eric Saward. Eric was enthusiastic, so Turlough was born. We decided that initially only the audience would be aware of Turlough's misdemeanours and shortcomings. This was to be followed by his fellow companions, most notably Tegan, being aware that all was not what it seemed in relation to the public school-boy from Brendon School. Finally, we were to reveal that the Doctor was no fool with regard to his quietly-observed feelings about Turlough and that the latter would finally choose good rather than evil when faced with the ultimate choice. There's only one thing wrong with all that - it was a little too cosy. "Evil companion turned good" would reduce Turlough's character to being rather banal, and so it was agreed that even after Turlough had proved himself to be worthy of his position in the Tardis there would be occasional glimpses of a large question mark looming over the character. Much more interesting, I'm sure you'll agree.

Turlough is one of my favourite companions in the show's history and I think it was chiefly his unpredictability that for me made him so. Often, as mentioned earlier, after an initially interesting character, mediocrity sets in in terms of development. With Turlough we had freedom, he had no set way of reacting because he had never fallen into the companion mould.

Much credit is due to Mark Strickson's portrayal of Turlough, but I nearly didn't get him.

When I was endlessly auditioning prospective "Turloughs", I received a phone call from Mark's then agent Jan Evans telling me she was absolutely convinced Mark would be ideal.

On the arranged day of Mark's audition, Julia Smith, who was then producing Angels, came to see me, as she had a major problem. One of the leading actors in Angela, who was playing an ambulance driver, had fallen ill and had had to be removed from the production.

Mark had played his friend and co-driver in a few episodes some weeks before and Julia felt that with some rewriting Mark could take over the storyline of the unwell actor on a long-term basis. Julia consequently said there was little point in my auditioning him as she was offering him a firm job. However, I insisted that I should proceed as planned. Mark was the last but one actor I saw for the part and by far the most splendid. I offered him the part later that afternoon, much to the chagrin of Julia! Mark couldn't believe it, earlier that day he was out of work and by late afternoon that eventful Friday he was being offered two years' work by two different producers. We gave Mark the weekend to think about it, as after all, ultimately it was his choice and I was delighted to hear on the following Monday that Mark had chosen Doctor Who. Apparently he had made his decision after being knocked off his bicycle by a lorry! I wonder who was driving the lorry!

I particularly liked the line-up of Tegan, Turlough and the Doctor - it seemed to me that the slightly sinister Turlough and the loud-mouthed Tegan created an interesting chemistry, out of which the fifth Doctor blossomed.

Mark was a wonderful company member, always smiling, charming and hard-working; also, in some ways, a very gentle person, who regards his private life as just that. I am openly keen on all manner of publicity - it is an essential part of the process of making TV programmes. Publicity makes the public aware that a twenty-three-year-old show is still running, for example. Because of my keenness in this area, I remember Mark coming to see me to let me know that he was getting married to the lovely actress Julie Brennan and "Please, John", he said emotionally, "no publicity!" His wishes were respected, of course.