You have a hell of an opening sequence involving flash floods, a baby rescue and life and death decisions. Was that what grabbed you when you read the scripts?

I opened the front page and started reading and I kept going, ‘Is that me? Is that my character? Do I get to do that? That is so cool’. Especially for someone in my casting bracket. I’ve done a lot of corsets and a lot of holding babies and mooning after someone in the background. To be doing the stunt and not just witnessing it was so exciting. I think it is so clever of Mick Ford to write that as the introduction for your protagonist. This is the world. These are the stakes. Here she is. She’s in the water. She’s saved the baby and she’s pregnant. Boom. You’re in. How could you not watch that?

How was filming the flash flood?

We were up in Stockton-on-Tees for a week, and it was like being on a school trip. Before that we had been filming in and around Manchester, so this was the only time we all went away somewhere. We were all giddy and excited. It is where they train all the emergency services. There’s this water course and they have control of the speed and the scale of the water. They had all these big strapping six-foot lifeguards in all their emergency gear stationed along the water course who were lovely. They said, ‘If you fall the water is going to take you but don’t worry, we’re going to save you.’ Luckily, I didn’t ever stack it, so they didn’t have to save me. It was a really exciting moment as an actress and as a woman in this industry to be the hero reaching out for the baby dressed in all my police gear with the rain going. The water was really strong, and we were in and out of it all the time. On the last day I realised my hand was shaking because I hadn’t eaten anything. I was so pumped from the adrenalin. It wasn’t a normal day at work where you do your lines, and you go home. It really made the job for me.

How do you look after yourself through such a full-on shoot?

I have so much admiration for people who lead these shows because it does require you to have your stamina up and be strong and be in every day and be concentrating on where you are in the story. Filming days are long and they’re hard and I was totally knackered. I just took a lot of vitamins and then I went away to Spain at the end of it and drank a gallon of white wine.

Where is PC Jo Marshall in her life?

We know that she’s pregnant and I think that any person who is pregnant can relate to that feeling that you know this monumental change is on your imminent horizon. It’s exciting but it’s also terrifying. You have this feeling that you need to get stuff done before it happens, especially if it’s your first. Jo is starting her detective training course, which is a big step for her. She feels that if she can just get the detective training done before having a baby, then everything will be fine. She is worried what life will mean post-baby and as a mother if she doesn’t complete the training beforehand. She has this quite restless energy about her anyway. Then the mystery that opens the show occurs and she just can’t help herself. She knows she might be able to piece this together which will cement her as a detective. Then she can go away, pop the baby out, come back and get on with her career. (laughs) Easy!

What is the locked-room mystery that Jo discovers in an underground car park?

Two days after the big flood Jo gets a call to go to an underground car park where somebody thinks there is a body. She’s first on the scene and this underground car park has flooded. The lift has jammed, and a person has been caught in the lift. Now the flood water has receded they’ve found this body. They think they understand what’s happened but when the autopsy comes back it turns out that this person actually died before the flood. So, the mystery begins.

Does Jo feel her husband and fellow police officer Pat is overly protective?

She does feel that, and he is, but it totally comes from a place of love. For Pat, it’s something that I think a lot of fathers will recognise. You are about to become a parent as well but it’s your partner that is carrying the baby. You are very much on this journey but also on the periphery. Everything that Pat loves about Jo – this slight recklessness and her persistence and doggedness - is also making him very nervous now. He’s just trying to protect her, but he is a bit clumsy with that sometimes. Jo loves him for it but when they start to butt up against each other that’s where the conflict comes. When you’re pregnant you are more than just a vessel for this child. There are all these things going on between them that I think so many parents will relate to.

What did you and Matt enjoy delving into about Jo and Pat’s dynamics?

It’s an interesting one because we’re quite private so we were saying, ‘Are we being quite hypocritical if we want privacy and then we do this job together?’ It’s tricky to know how much to share about it. What I will say is we really like working with each other, are similar in the way we work. Both of us are very much you do your homework privately, turn up, do the job, go home, get on with it. We are not actors who individually are like, ‘How do we relate it to ourselves?’ Jo and Pat are nothing like us. It was just a really nice experience of being able to go to work with your partner. It’s so funny because normally when you are on set, and you’re not needed you go to the green room. You are with other actors, and everyone sits, and they chat. But when it was just me and Matt, we would take ourselves off to the green room and just sit in silence and read our books and play on our phone because it was such a relief. Our son is two so this was like a holiday going to work.

Does it make any difference if there are scenes where Jo and Pat are arguing?

There was only one day where our characters have a row. It was quite early on and apparently when we came home, I was in a terrible mood. I refuse to believe it. Matt said to me, ‘You know we didn’t have a row today? Jo and Pat had a row.’ I was like, ‘Oh yeah, I think I was angry at you for something Pat did!’ That was the only time that happened.

What’s Jo’s relationship like with her mum and what have been your favourite scenes to play with Lorraine Ashbourne?

I am completely besotted with Lorraine Ashbourne. Everybody is. She is so cool. She’s got this amazing energy. When we met, I was like, ‘Oh, this is totally going to work.’ I don’t know what it is, but she is just effortless to do a scene with. It was so fun, and they are brilliantly written. Mick has really understood that relationship with Jo and her mum, they are borderline sisters at times. The way they communicate is by rowing with each other. We did this lovely scene where we come into the kitchen having a row. I’m telling her off and she’s telling me off and it’s all love. It’s so clever. It’s two people saying one thing meaning something else. Jo’s domestic side with Pat is quite strained. There is a lot at stake and a lot they’re not telling each other. But Jo’s relationship with Molly is effortless and easy and fun and warm, so it was nice to have those scenes to counterbalance the high stakes scenes. I just loved it whenever Lorraine came in, she’s an amazing raconteur and has hilarious stories. The crew love her, she’s brilliant to have on set.

What’s the little finger ritual that Jo & Molly have?

Mick wanted just a silly little ditty that they do together. They link fingers and sing this silly song. It’s the sort of thing you imagine Molly’s been doing with Jo since she was about six. So, we had to figure out what that was. I don’t know what it is about Lorraine’s energy, but she makes me very giggly. Every time we did it, we’d giggle and then the fear that we’d get it wrong made us giggle even more and that’s what Mick intended. It’s such a small detail but a clever way to tell the audience that this is a really good relationship.

What does Jo think of her sister-in-law, Kelly played by Faye McKeever?

Faye is hilarious, she cracked everybody up. The way she played her at the read through she just barrelled into scenes as Kelly, loud as anything and telling people how it’s done. Kelly drives Jo to distraction; they are total opposites. Kelly has no self-awareness and no tact. Because of the flooding, Kelly and her family come and live with them. That’s grating against Jo and all her nesting instincts and her need for privacy, drawing out another side of Jo. She gets a bit more Northern when she interacts with Kelly and fires it back at her. You want to see that side of Jo as well. All these interactions draw out who is this person we are following through this story and that’s an important aspect of Jo.

How did you and Tripti Tripuraneni develop the working relationship between Jo and PC Deepa Das?

It was great because me and Tripti were in the flood sequence together including the first lot of rain sequences, so we felt right in the trenches with each other. We were two mums having an absolutely lovely time dressed as police officers thinking we were marvellous. By seeing how Jo interacts with Deepa you understand how Jo is on the beat, how good she is in the community, and you can see the detective she is going to become.

Is it fun to play the more experienced police officer having been the rookie back in season one of Happy Valley?

Yes, this is what PC Kirsten McAskill could have been maybe! (If she hadn’t been murdered by Tommy Lee Royce). It was fun to be the different person in the scene. It was fun now being the one figuring out the mystery.

What have been your favourite locations on this job?

What was amazing was the Colliery Road set. After the flood Jo gets a call to go and help. She walks down this flooded residential street. You read it and you think, ‘How are they going to do that?’ They built it round the back of the Trafford Centre in the car park and then they flooded it. It was essentially a huge water tank with two pretend houses on either side. You would get into the water tank, and everybody had waders on up to their chests and you would wade around. At one point the DoP got in a boat so he could use the camera more efficiently, it was hilarious. I’ve never seen them build something on that scale in the British TV that I’ve worked on. It was real commitment; they added all these incredible little details like it was people’s real lives and their houses. All the curtains in the windows and the shop front and little kiddy toys floating or a shoe. After the flood water has receded people are looking at the damage to their houses. That was quite startling because you were really confronted with the reality of flood damage and the reality of climate change. This is a real threat to people. It was a really extraordinary set, and it really represents a lot of what the show is about

If your house flooded, what would you save first?

I would probably save my little boy’s favourite stuffed toy. It’s a little dog and he can’t sleep without it so actually it’s a selfish choice. I just want him to have a full night’s sleep.