WARNING: SPOILERS. DO NOT READ IF YOU HAVE NOT SEEN SHERLOCK SERIES 4 EPISODE 1

In one of the final scenes of New Year’s Day's Sherlock series four opener, John Watson cradles his dying wife Mary as she whispers her last, poignant words to him.

It's a heartbreaking moment, increased tenfold by the fact that the two actors at the centre of it – Martin Freeman and Amanda Abbington – were partners in real life.

So how did the cast deal with that emotional showdown in the aquarium? According to episode writer Mark Gatiss, one strategy was to spend the day’s filming making up facts about the sharks that were swimming around them, such as “sharks don’t eat beans” and “sharks can’t spell”.

But for the rest of the time it appears they were concentrating on the depth of emotion in the scene – and not screwing it up.

Gatiss said Freeman “blew the mic” with the guttural wail of grief made by his character when he realises that Mary has died. Freeman said that he improvised that howl of anguish in the moment.

“That was all me baby,” Freeman revealed at a Q and A after the screening of the episode. “It’s difficult. You are kind of always – I am – always on the verge of acting badly. You work up to a point where you could get caught out acting badly, especially where it’s your wife on telly and who we are in real life. You have to do it justice obviously, but it’s very easy to overdo it. It’s a careful line to walk. 'And… act that Amanda’s just been shot'. That’s pretty difficult. That’s pretty tough even when you know it’s coming in the schedule. It's pretty difficult."

Benedict Cumberbatch joked “it was a walk in the park" before adding: “It was emotional. We get the hit that the audience hopefully gets when they watch it when we first read it. So that was always going to be a moment in the schedule, and it was. It was a big moment.

"Three became two and this incredibly important part of what Sherlock is, is suddenly no more, in the most violent way imaginable. And a mother as well as a wife and a dear friend and brilliant, and you discover more about her just as you lose her as well. It was a great bit of tipping someone over the edge of a cliff as soon as you are most in love with them. It was a very upsetting scene."

But it was the departing Abbington who appeared to have most enjoyed filming the scene.

“It was good. It was sensitively handled. It was nice working with these guys. It’s always nice when all you get from the script [is] it says you get shot, you say these last things and it was a dream as an actor because you can just be completely self-indulgent, you can really go to town on it. I probably did a bit. Doesn’t matter does it? It was fine. It was good fun. I was looking forward to it. I got to over-act.”

The next episode of Sherlock, The Lying Detective, is on BBC1 on Sunday 8th January