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British actor Benedict Cumberbatch has been hot stuff ever since he started playing the titular sleuth on BBC TV drama Sherlock (2010 - present).

But what if the 39-year-old becomes such a big deal that he decides to leave Sherlock altogether?

Pose the question to Mark Gatiss and Steven Moffat, the co-creators and writers of the show, and they have the perfect response.

Gatiss, 49, says in jest: "I worry about the day when we're too expensive or busy to come back to the show."

Moffat, 54, laughs and says: "Yes, what about us?"

In fact, they are rather "disgusted" that Cumberbatch has gone on to star in blockbusters such as Star Trek Into Darkness (2013) and The Hobbit: Desolation Of Smaug (2013). He will also star in the upcoming Marvel superhero movie Doctor Strange.

The same goes for Cumberbatch's co-star Martin Freeman, who plays Sherlock's work partner and friend Dr John Watson. Freeman has since gone on to play the leading man in The Hobbit film trilogy (2012, 2013 and 2014).

Moffat quips: "Mark and I are very open in our disdain that they cheerfully go and work on other people's projects for more money and fame. We are disgusted and genuinely offended by their faithless nature."

On a more serious note, Gatiss says: "The fact is that Benedict and Martin are already too expensive and busy for us, but they come back because they love the show."

The two actors have confirmed their return for the show's fourth season, slated to air next year .

Throughout the 15-minute telephone interview with Gatiss and Moffat from Sydney, Australia, where they are promoting the show, it is clear that the two British showrunners share a sizzling chemistry, often finishing each other's sentences if they are not poking fun at each other.

Much of that wit carries over into their screenwriting, as evidenced by the rapid-fire banter between Holmes and Watson on the show.

Gatiss has an acting role in the series as Sherlock's smart older brother Mycroft Holmes.

The pair of self-professed "very good friends" are also writers of the popular time travel TV drama Doctor Who (2005 - present). But it was only for Sherlock's most recent episode, The Abominable Bride, where the two finally sat down to pen a script together. In the past, the show's episodes had been helmed by either one or the other.

The Abominable Bride is a one- off special episode that takes Sherlock from modern-day England back to 19th-century Victorian London, where the original Sherlock stories by Sir Arthur Conan Doyle were set.

Gatiss calls the episode a "treat" for fans, adding that the drive to make it was "irresistible".

"We had joked about the idea for a long time and when we got the timings for everybody and realised that it was possible to do a special like this, we saw it as a grand opportunity and went with it.

"The idea of seeing Benedict and Martin and everybody else in 'Doyle land', as it were, was just massively appealing."

1 Should Benedict Cumberbatch and Martin Freeman ever decide to leave the show, would you consider recasting their roles?

Moffat: No, no, no. There wouldn't be any point in continuing without them. Our version is those two men in those two parts. If either one of them decides he has had enough, then that's it - absolutely.

2 The Abominable Bride is the first time the two of you sat down to write a script together. What was that experience like?

Gatiss: It is the first time we did it together, but actually, the episodes are always thought through together, even if we don't sit and type in the same room.

Moffat: Technically, it's the first time but it didn't feel like the first time.

3 What was the cast's reaction when you first approached them with the idea of doing this Victorian-set episode?

Moffat: Well, actors always relish the opportunity to dress up and do something different from the characters they have been playing.

Gatiss: The special is like a palate cleanser. We had a lot of fun going back to the way the stories were when we first fell in love with them.

4 Will this special episode lead to more episodes in other settings?

Gatiss: I don't think so. We're writing series four, taking things back to the modern-day setting. We've joked about doing a black- and-white one where they fight the Nazis. So, who knows? Really, though, this was a fun side-step but it'll probably be a one-off.

5 Do the two of you socialise outside of work?

Gatiss: There is no such thing as "outside of work" (laughs).

Moffat: We do, in theory, socialise, but we end up talking about Sherlock anyway.

Gatiss: We had drinks a couple of years ago and the whole point of the night was not to talk about Sherlock or Doctor Who. We said we'd talk about James Bond instead, but by the end of the night, we had formulated an entire James Bond movie.

6 Fans swarm the Sherlock set whenever shooting takes place in public London locations. Has that been difficult to deal with?

Moffat: No, the fans are so lovely when they come to the set. Of course, it's a little unreal when you have crash barriers when we are shooting on the streets with huge crowds of people staring at us, but they all behave beautifully. They are very quiet during takes and they are anything but trouble.

7 British shows are gaining popularity around the world, while many British actors lead major American TV show casts, such as Game Of Thrones. What do you think of that?

Moffat: It's a stealthy invasion. We are conquering the world through the media. We have an army ready to be deployed at any moment, led by Benedict and with Liam Cunningham as his lieutenant.

Gatiss: And then we declare ownership of the world.

8 How would you like to be remembered?

Gatiss: Like Woody Allen, I don't want to achieve immortality through my work. I want to achieve immortality by not dying.

Moffat: Yup, we would like to never be remembered - because we're still around.

•Follow Yip Wai Yee on Twitter @STyipwaiyee

•Sherlock: The Abominable Bride is available on BBC First (StarHub TV Channel 522) on demand.