Series eight opens with a time jump forward, with Bonnie and Will about to have a baby. Tell us where we find Will at the start of the series.

Will is a happily married man; he and Bonnie couldn't be happier. They’re best friends. But Will is struggling a little bit with disciplining [his stepson] Ernie and being a tougher parent, because obviously his father was the worst kind of parent and he wants to be the opposite. So Bonnie gets frustrated a little bit, saying, ‘You need to be a father. You need to properly tell him what to do. Otherwise, he'll be trouble.’ So there are teething issues with that, but, all in all, Will is loving life, until something happens that takes Bonnie away and then leads to it all coming crashing down.
So Will is beset by this great tragedy in which somebody loses their life. How does it affect Will?
From the moment we first met Will, he’s always been struggling with his demons. What drives him in life is being the best man he can – everything he does, every case he helps Geordie with, every problem that he solves, he feels like he's getting better and better as a person, morally. And then this thing happens that just seems to erase all of that in one instant. Everything he’s worked so hard for is gone just like that. It just destroys the way Will thinks of himself, because this action, whether it was an accident or on purpose, in the eyes of God and everyone else, he believes it’s unforgivable, and it really sets him down a dark path.

It leads Will to have a crisis of faith. What can you say about that?

Now that this thing has happened, he's questioning whether God or anyone can ever love him again. He just shuts them out and it pushes him further and further away from everyone to the point where we don't know if he's ever coming back.

How does Geordie try to help?

A couple of series ago, Will was helping Geordie when he was going through his darkness. Will was there for him and really tried to pull him back to the light. And so Geordie does the same for Will. He's his best friend and his father figure. Geordie understands the guilt that Will feels, because he's gone through it himself. It doesn't mean that Will listens. The tragedy of this series is that everybody thinks they’re losing the Will that they love and Geordie has to watch it happen while also trying to deal with his own issues with his job and possible retirement. Geordie doesn’t realise how far down the path Will goes.

You and Robson seem to have forged a close friendship offscreen as well as on. What can you tell us about that?

We are best friends. Genuinely, we have a little bit of an age difference, but it doesn't exist when we’re together: we're both children and best mates on set just messing about. We laugh all the time. But also, outside of work, we always try to see each other. I go up to his house in Northumberland sometimes just for a sleepover, to hang out with him and his partner. We’ll watch Jackass on TV and eat sweets. I love it and I hope we’ll always make that effort.

What can you tell me about the cases this series?

They’re a lot more personal this series than before. They’re not just strangers within the Grantchester community who are getting murdered; they all have a link in some way, which then compounds what happens to Will even further, because it's seeming like he just has the mark of death upon him. He feels he is a corrupt individual now who is only going to cause death and misery. Whether he actually has any responsibility at all for what’s going on, he's making it his problem. It’s different from previous series, because normally there's a distance between Will and the murders; he can detach from them emotionally a bit more, but that becomes impossible for him to do.

You’re working with real babies this season as well as Dickens the dog. How was that?

I both love it, and it has also made me put a pause on thinking about being a parent for a little bit. There are a couple of babies that I’m holding this series and it made me so broody. They’re lovely. But, also, when it’s not your baby and you have to do 30 takes with this baby that quickly gets very bored and starts to scream, it’s not so much fun. Will’s dog, Dickens, was very well behaved this series, although in every scene with Dickens I probably have a whole handful of sausages in order to bribe him. He’s got a very good agent, while I get greasy hands and have to change my costume.

How do you think Will has changed over the five series you’ve been in?

What’s sad is that this series, when we meet him at the beginning, he's really the best man that he's ever been and he's worked so hard to get there. The thing you love so much about Bonnie is that she knows him so well and keeps him grounded. I think he's matured a lot and is starting to shed the demons that were pulling him back. He will always have the issues with his father and his upbringing, but he can talk to Bonnie, who helps puts things in perspective for him. So I think he's matured.

The motorcycle has been a big feature of Will’s life. Can you tell us about that, and what it’s like for you riding the motorbike?

The motorbike has always been a symbol of Will’s rebellion and something he's holding onto from his youth. But he’s a family man now and when Ernie sees Will come off the bike in episode one, it makes Will start reassessing his priorities. Then he has a bit of an immature wobble: he doesn't take some news very well, goes back to old habits slightly, which then has a knock-on effect for the entire series.

I love riding the motorbike so much. I was so scared when they made me get my licence, but the minute I got it I understood why people love motorbikes. The producers won’t let me go very fast, but my stuntman makes me look very cool on that bike. I briefly considered buying a motorbike when I got my licence, but I wouldn’t want to put my parents through the worry.

How does the cast get on behind the scenes? Is there a lot of joking around?

Usually, it’s me and Robson. For example, I have these jelly beans that are very spicy, but they look like just strawberry flavoured jelly beans. Robson isn’t very good with spice, so just before we did a scene I said, ‘Do you want a strawberry jelly bean?’ It was a ghost pepper jelly bean. So he was doing the scene and his eyes were absolutely streaming. He was crying! It was so funny.

You directed your first episode last year. Will you direct another?

This year Al Weaver [who plays Leonard] directed an episode. And yes, of course I took the mickey out of him, because he did the same to me when I directed! But when you direct you can’t be on camera as much, and Will’s storyline is quite intense this series, so they needed me onscreen. But I loved directing. It gave me such a thrill working with all these actors that I admire, and they were so professional and made it so easy for me to do.

What do fans say to you about the show?

I get told off quite a bit! They say, ‘Will had better look after Bonnie.’ I say, ‘I’m not actually Will, but I promise I’ll pass that on.’ And people say, ‘I wish you wouldn't smoke so much,’ but it's the 60s and everyone did smoke quite a bit back then. But people really care and I'm excited to see what they make of this series. I love seeing people on Twitter really getting invested in the storylines.

What do fans get from watching Grantchester?

It's more than just a murder mystery, which sets it apart from other long-running shows. It's about these characters that you care so much about. Fans love to see our characters involved in this dark crime and they love the scenery, the fact it’s filmed in this idyllic part of the countryside. It looks so beautiful on screen. The thing I'm most proud of being part of this show is that the Grantchester writers make us really care about the characters.