Dirk Gently (Stephen Mangan) and Richard MacDuff (Darren Boyd)

The novel writer is master of his domain. Apart from odd suggestions from an editor or publisher he is king, supreme leader and dictator.

Making television is essentially collaborative and involves the creative input of a huge number of people, all of whom, to a greater or lesser degree, influence the finished product.

The producers are responsible for hiring everybody. They choose a writer (or writers in our case) to produce scripts. These scripts will set the template for all that follows but there are thousands of creative decisions still to be made.

A director is hired. Who you pick will have a massive impact on the finished product because nearly all decisions from here on in are made in conjunction with him or her.

Actors are chosen. They picked me to play Dirk but imagine the show with, say, Damian Lewis or Harry Enfield or Alexander Armstrong as Dirk instead. (If you believe any of them would have been better than me, please don't tell the BBC.)

Darren Boyd plays Richard but how different would the show be with Rupert Penry-Jones or Russell Tovey as MacDuff?

Locations for filming are chosen with the help of a locations manager, helping to find the perfect place as described in the script and if you can't find the right place you might build a set, which has to be designed, built, decorated and furnished.

The art director is responsible for the look of the whole show and, with the art department, decides everything from the colour of the walls to the knick-knacks on a shelf.

The costume designer clothes the actors. My look in the series is different from the pilot because we had a different costume designer who brings her own sensibility.

Neither is 'right' but they are both different and have an impact on what the audience feels about Dirk. Imagine David Tennant's Doctor Who in a bomber jacket and DMs.

The cameraman and the director chose the style of shooting. Again you'll notice a difference between the style of the pilot and the style of the series. Which do you prefer?

Look out for the very first sequence in the first episode of the series, up to the title credits, and imagine quite how many decisions had to be made about a myriad of things just to put that together.

Then there's the music, which has an enormous impact of the feel of a show. Hair and make up, lighting, sound and dozens more, hundreds of decisions have been made about hundreds of things that all impact on what you see on your screens.

Unlike books, which require a writer, a laptop and a reader to do all that work in their heads.

Stephen Mangan plays the title role in Dirk Gently.

Dirk Gently starts on Monday, 5 March at 9pm on BBC Four. For further programme times, please visit the episode guide.

Comments made by writers on the BBC TV blog are their own opinions and not necessarily those of the BBC.

Dirk is described as "a pudgy man who normally wears a heavy old light brown suit, red checked shirt with a green striped tie, long leather coat, red hat and thick metal-rimmed spectacles".

Well, I'm a man; we got that much right. But I'm not that pudgy and I play Dirk wearing none of the clothes described.

Does that matter? Should they have scoured the country for a chunkier actor? I believe there are some out there.

Without the red hat is the whole enterprise doomed to failure? Is playing him without a green striped tie tantamount to dancing on Douglas Adams' grave?

There are still people out there furious that James Bond is being played by a man with blond hair.

A blond Bond? The books say he has black hair falling down over the right eyebrow!

It's a thorny old question - how slavishly should a screen adaptation follow the book?

Some people won't be satisfied unless the images they had in their head whilst reading the novel are translated exactly onto the screen.

But what most people want, I imagine, is that they enjoy the screen version as much as, if not more, than they enjoyed the book and that the spirit of the book is preserved - if not the thick metal-rimmed glasses.

In my opinion Dirk Gently's Holistic Detective Agency and The Long Dark Tea-Time Of The Soul are unfilmable as written.

You couldn't begin to do justice to them in a single hour of television. Too much happens, there are too many ideas.

So Howard Overman, our writer who knows a thing or two about writing for television, as any of you who watched Misfits will know, decided that if he was going to write an hour of telly then it needed to work as an hour of telly first and foremost.

It sounds obvious but you'd be amazed how often that isn't the priority.

Once that's established and you realise that you can't shoehorn the whole book into that time, you've got some decisions to make, what's in and what's out? What do we need to invent or add to make what's in work?

Once everyone's happy with the script, you cast it. Again, does it matter that I don't look like the Dirk that's described in the books? Is it enough that the actor gets the spirit of the character?

Dirk is one of the most interesting and complex characters I've played. He's charming, irritating, bright, funny, hapless, unreadable, transparent, roguish, chaotic, philanthropic and possibly dishonest.

If I get all that right, am I allowed to be too thin?

Television is a team sport, novel writing isn't. Our film has creative input from Howard, me, the director, the producer, the rest of the cast and dozens of others.

And all the stuff from the books that doesn't feature is still sitting there ready for us to use once the BBC commissions a 58-part series...

I'm extremely proud of how it's turned out. I hope you enjoy it.

Stephen Mangan plays Dirk Gently in Dirk Gently.

Dirk Gently is on BBC Four and BBC HD on Thursday, 16 December at 9pm.

Comments made by writers on the TV blog are their own opinions and not necessarily those of the BBC.