In the world of film, the director is king and the writer is merely a humble servant. Screenwriters may fill blank pages with bright ideas, unforgettable characters and pearls of sparkling dialogue but, more often than not, the director is considered the author of a particular work. It is a situation that Elgin-born Allan Shiach finds particularly galling.

"The world was given a wonderful play written by Robert Bolt called A Man For All Seasons," he explains. "It triumphed around the globe and then Robert Bolt adapted his own play into a screenplay that became known - by law - as Fred Zinnemann's A Man For All Seasons. That is wrong. We don't talk about Simon Rattle's Ninth Symphony and I think one day that balance between the writer's primacy in this medium and the director's will be more recognised."

Using the professional name of Allan Scott, Shiach has almost 40 years' experience as a writer. His career stretches from a fertile collaboration with director Nicolas Roeg on classic films like Don't Look Now (1973) and The Witches (1990), to his current triumph as the co-writer and co-producer of the stage musical Priscilla, Queen Of The Desert. At Aberdeen's Word Festival next month, he will discuss the craft of adapting a novel for the screen, a subject he can speak on with some authority as someone who has adapted the work of Roald Dahl, Pat Barker, Henry Fielding and Alastair MacLean, among many others.

"There are two important things in adaptations," he advises. "You are taking a 500-page book and turning it into a 100-page script, so the first thing is editing; you simply have to cut to the core. The second thing is to decide for yourself what are the really important values in this book that make you want to adapt it. You have to absorb those important values - whether they be narrative, ethical or even contemplative - into your thinking process and find ways to use them in your adaptation. You don't have to be literal about it but you have to be honourable about it. I think there is a huge amount of honour involved in adaptation."

Shiach cites Gone With The Wind (1939), The Age Of Innocence (1993), The Shawshank Redemption (1994) and the recent Oscar-winner Slumdog Millionaire (2008) as textbook examples of screen adaptations. "All for the same reason," he adds. "The writers took essentially literary material and held on to the emotional truths of the novel while making them into a pure cinematic experience. A nice trick."

Shiach's adaptation of Daphne Du Maurier's short story Don't Look Now probably remains his most celebrated screen work. A chilling psychological thriller set in Venice, it fragments time and narrative to striking effect as it tells the story of a man haunted by the drowning of his daughter and the foreknowledge of his own death.

"Chris Bryant and I were asked by a producer if we would like to do an adaptation of the short story," he recalls. "We worked with a couple of directors who came up with a couple of additional ideas. Those directors fell by the wayside and then one day it was read by Nic Roeg, who said he'd like to do it. He reads the script and then he sits down with you and goes through it page by page. You discuss everything. The opening scene of the child's death was in the script long before we gave it to Nic, but then he makes it his own because he's that kind of director."

Shiach believes that, as a general rule, writers are never ideally placed to adapt their own work. Objectivity is impossible and destroying the thing you love the most can be beyond them. Du Maurier, for instance, had nothing to do with Don't Look Now. "I understand she thought it was an extremely accurate representation, which just shows how well she knew her story," declares Shiach. "Her story was about a couple whose child died of meningitis and the husband did not work as an architect in Venice."

Barker was happy to offer Shiach advice and support when he adapted her first world war novel Regeneration. Dahl was rather less supportive of Shiach when he adapted The Witches.

"I was unbelievably faithful to his book and he became completely curmudgeonly about it all," Shiach recalls ruefully. "He said to me at one stage, Why did you have the parents die at the beginning - this is a children's film'. I said, Because, Mr Dahl, in your book the children's parents die.' He had completely forgotten."

The former chairman and CEO of the Macallan Glenlivet whisky company, Shiach found his business expertise armed him for the challenge of turning producer later in his career, although he never wanted to direct. "I don't have the patience to sit in a darkened room and look at the same scene 48 times in order to trim three seconds out of it," he confesses. "I do not have the patience to go and knock on the door of the star's caravan and beg them to come out and deliver the line as written. Not for me."

Shiach, 68, has recently been devoting a lot of his energies to the theatre. He has co-written the thriller The Conjuror, which will be staged in 2010, is working on a multimedia event set around the music of the second world war and is also planning a show based on classic revues from the 1950s and 1960s. The dulcet merriment of Flanders and Swann could be discerned in the background at the start of our conversation.

Priscilla is now set to tour the world with productions planned for New York, Toronto, Germany and Scandinavia. Shiach is a devotee of musicals, although he plays down the possibility that this has been a major change of direction for him.

"It's all just writing," he says modestly. "Whether it's a musical or a play or a film, it's all about solving puzzles. How do you expose that aspect of a story without making it look obvious? How do you reveal this aspect of a character without hitting it over the head? It's all those things."

Shiach remains committed to cinema, detailing one current project that may yet see the light of day. "I've got a script that I launched on the world recently and the thing I like best about it is the pitch - Pulp Fiction meets It's A Wonderful Life. It's set in modern Los Angeles and is about a terrible gangster and a black angel who comes down and tries to redeem him before he dies. The angel is called Clarence and I describe him as looking like Puff Daddy on speed. A few weeks after I sent it to my agent in LA, I was asked if I would speak to a guy who wanted to finance it and play the angel. It was some time before I realised I was talking to Puff Daddy, as they introduced him as Mr Combs and I never made the connection."

Shiach's dream project remains an adaptation of the 1983 Walter Trevis novel The Queen's Gambit, which is set in the 1960s and tells of a teenage chess prodigy whose career may be hampered by her self-destructive impulses.

"I have owned the rights for some time and worked on the script with various different directors including Michael Apted, Bernardo Bertolucci and Tom Tykwer," he reveals. "Last year, I wound up with a first-time director who absolutely blew me away. We worked on the script and became very close friends and we were in the process of casting when he died in New York. His name was Heath Ledger, and he would have been an excellent director."

Allan Shiach: From Novel To Screenplay takes place on Sunday, May 17, at 1.30pm at King's College Centre, King's College. Aberdeen. Priscilla, Queen Of The Desert is at the Palace Theatre, London.