Actress Jennifer Hennessey knew from the first day of rehearsals that she was Jacob Marley’s killer, the plot at the heart of BBC1’s Dickensian, but had to hide it from the cast and crew.

The stunning twist in episode 17 on Friday saw Inspector Bucket (Stephen Rea) deduce that the fragment of wood found in Marley’s head actually came from pie-maker Emily Cratchit’s rolling pin!

Only Jennifer and Stephen knew the identity of the killer from the start, which meant she spent the entire shoot hiding it from her friends and colleagues.

What’s on TV caught up with Jennifer to talk through Emily’s means, motives and opportunity…

How did you feel when you learned Emily was twisted money-lender Marley’s killer?
“Well, it was a shock to me when I first found out because I thought this was a straight-forward, loyal working-class mother and when I found out on the first day of rehearsals I was, ‘OK let me get my head around that!’ It was entirely what I wasn’t expecting. But really exhilarating actually because every single scene I read is about something else… I had to think about the subtext, which is really challenging and exciting as an actor.”

Not your average role, then?
“It was different for me because of all the subtext and things that were happening underneath, because I knew I was part of this really special storyline.”

Why did the producers tell you, but keep it from the rest of the Dickensian cast?
“I think to give me a good go at it, really. So that [I could] do it truthfully. I did have this conversation with the Cratchit family, all the actors were stood around saying, ‘Is it a better thing to know or is it not a good thing to know?’ and having to join in the conversation, ‘Oh I really don’t know!’

“But what was clever about knowing was it meant that I was acting off-set as well as on-set, having to get used to telling half-truths to actors and crew, so it kind of got me in the zone in a way. If no one knew or suspected then it meant that I was getting the level right, that I wasn’t giving anything away or over-playing, so it was quite a useful exercise.”

Stephen (Bucket) was in on the killer’s identity, too, but no one else. Why?
“It was useful for the other actors because nobody knew if it was them so when they were being questioned by Inspector Bucket they could possibly… for all intents and purposes it could have been Scrooge, so when he was being asked questions there was that underlying suspicion because he didn’t know himself whether it was him. So it was quite useful for everybody.

“But it did link me and Stephen strongly in the whole process, which is right, because [Thursday’s] episode is his dilemma given that he understands this woman’s plight, so I think the whole thing was really clever from the producers to make that decision.”

We know there’s more to come in terms of her motives, but what can you tell us about why Emily killed Marley?
“She wants to confront the injustice of what’s happening to her and Bob [Cratchit, husband], because Scrooge and Marley keep undercutting their wage, this is in a time when if you’re down a few shillings in a week it’s the difference in being able to feed your children or not. It’s desperate, really…”

Dickensian has been carried off with aplomb, but on paper it looked like a risky drama to make…
“I think if you’re a Dickens purist you would have had reservations definitely… But [creator Tony Jordan’s] actually just taking plots that were in the margins of Dickens’ novels and run with them and explored them and made them more exciting. To the point where people who haven’t read Bleak House will want to go and read it now.

“I’m dying to see how Honoria turns out. I think it’s made people excited about Dickens as well as excited about Dickensian.”

It’s been a wonderful series, but things are looking bleak for all concerned. Will there be any happy endings?
“It doesn’t look like it, does it? But it’s Dickensian so anything is possible, isn’t it? Having said that I think [Tony Jordan’s] on track to get back to what’s happening in the novels so I think it’s a wait and see. All storylines are resolved in some sort of satisfactory fashion, so you won’t feel cheated by the end of it.

“It’s cleverly left open just in case they want to give us more series. There’s loads more to pay with, isn’t there? There are characters galore. I mean, I want to see [Great Expectations’ benefactor] Magwitch come into it. I think that would be really exciting…”

The final three episodes of Dickensian screen on BBC1 on Thursday, Friday and Sunday this week.

Watch a preview clip from Thursday’s episode, above.