Remember that famous scene in the film Heat where Robert De Niro and Al Pacino finally share a screen together? There's a cheerier hint of that at the Marylebone Theatre this Christmas, as two of literature's best-loved characters come face to face: Sherlock Holmes and Ebeneezer Scrooge, the great detective and the redemptive grinch. It's quite a moment.

A Sherlock Carol is Mark Shanahan's spookily comic and fiendishly clever festive fairytale, adding a whole new chapter to the lore of Charles Dickens' Scrooge and Arthur Conan Doyle's Holmes. Rather than yet another straight take on A Christmas Carol - Christopher Eccleston and Keith Allen are both starring in versions this Christmas - Shanahan opts for a sequel-cum-crossover; imagine Alien Vs Predator, but fun.

Holmes is the main protagonist - well, that theatre is (Scotland) yards from Baker Street - and he's faced with quite a mystery, should he choose to accept it: what did happen to Ebeneezer, post-ghosts? Ben Caplan (Call The Midwife, Band Of Brothers) is enjoyably irascible as the now-grizzled sleuth, probing Kammy Darweish's Scrooge - so we probed him about donning the famous deerstalker.

How did you first hear about A Sherlock Carol?

I was approached early on by a colleague of mine in New York about the production, to see if I would be interested in reading the script. When it finally was scheduled for a run last year I was asked if I would be interested in playing Sherlock Holmes. I had a chat with Mark [Shanahan] on Zoom and we discussed our mutual love for A Christmas Carol and Sherlock Holmes and the rest is history.

Was the script's tone set in stone when you joined last year, or did it become more comedic/dramatic along the way?

The play had been presented in New York but it was always Mark's ambition to bring it to London, and especially to a theatre around the corner from the Sherlock Holmes museum in Baker Street. I'm not sure what they discovered and presented in the US but I think we have brought a darker more melancholic tone to it. The fusion of the comedy and tragedy makes the perfect recipe for a Christmas theatrical treat.

How did you approach playing Sherlock? Did you reference any previous portrayals, or actively avoid them?

I was a big fan of the Sherlock Holmes stories growing up and used to watch the BBC series with Jeremy Brett. Since taking on the role I actively avoided any particular portrayals and went back to the original stories to pick any useful tips and references. I'm always very keen to try and find my own take on any particular character and use the original source material as well as making discoveries from what is in the script.

It's a fabulous idea, fusing these two worlds - although I suppose also a danger of infuriating twice as many people, if you hadn't got it right?

I think any fans of both Dickens and Doyle will really enjoy this festive mash up. As far as I'm concerned you get two for the price of one. All the feedback I have heard has been entirely positive and the really dedicated fans have really enjoyed all the references and easter eggs.

What's your favourite version of Scrooge, this production aside?

I have always been a huge fan of the musical film Scrooge starring Albert Finney. It has a brilliant cast with the likes of Alec Guinness and Edith Evans along with a brilliant performance from Finney in the title role. I watch it every Christmas and it has become a festive tradition in my household. I've really enjoyed introducing it to my kids.

Any interesting moments during the prep for the play? Is it a spookier atmosphere than usual when ghosts are involved?

Nothing that I can think of other than having a lot of fun in rehearsals and really enjoying discovering new things every time. The sign of a great script is being able to get something new every time you work on the text. I have yet to be haunted by my real life nemesis...