Can you tell us about your character, Moira?

She's wicked. She's the best kind of character to play, because you get to be funny and evil. It's always more fun to be the bad guy, so I love her. She's so horrible!

Why is she so horrible?

She's hugely bitter. I was thinking about the character when we started filming and I thought, "I wonder what she was like when she was younger?" and I decided she was extremely beautiful of course!

But I decided she was married twice before, so I wear two rings. She's angry and bitter and upset, so she has thrown herself into reading these murder mystery books by David, so she quite looks forward to Griff Rhys Jones's character turning up and showing them round. She's a bit of a fan.

What was it like, using the wheelchair?

Well, I only did this job because it was a free workout for my arms! It's quite unwieldy, and everyone was laughing at me, because when I had to do something uphill, I couldn't - and I started to roll backwards. That was hilarious, apparently - to everyone except me.

Do we know why she is in the wheelchair?

Well, my back story is that she can actually walk, she's just lazy. She just likes to order Laura around and make her push her everywhere.

So what's the relationship like between Moira and Laura?

It's all in Moira's favour. I'm like the evil stepmother, you know? Laura's Cinderella, and Moira's just really horrible to her. Nothing is good enough for Moira, no matter what Laura does to please her and be nice to her, it's just not enough.

Has Moira got any redeeming features?

Let me think... Moira is one of those people who will just never be satisfied with anything, so no.

What's it like playing her?

For some reason, Moira just exists in me - the minute the wig comes on, the corners of my mouth just automatically turn down, and I don't know if it's because the wig is so ugly!

You're almost unrecognisable.

It's brilliant. I cannot FaceTime my husband and children with this wig on, though, it's hugely upsetting for them. My husband refuses to look at me! He said, "I'll talk to you, but don't FaceTime me."

How would you describe the tone?

It's dark comedy. It's black comedy, that's exactly what this genre is, it's very black comedy, and it's exactly the kind of comedy I like. And the calibre of actors in it - it's unusual, because a lot of times you have a cameo by a couple of people, but this is ALL done by people who normally get to do cameos. So it's very ambitious.

I think anyone who watches crime stories and those kind of things, it's got a feel of an old-school Miss Marple, but with more modern characters, so I think people will enjoy guessing.

What does the coach setting add to the drama?

It gives you some claustrophobia. I think the coach element is always interesting - anything where people are in an enclosed environment is always good, like an aeroplane or a shed, or a lift. Anything where people are forced to be together that wouldn't normally be together - that's just comedy gold anyway. And the irritation levels go up and up and up.

Have you managed to keep a straight face during filming?

I've been guilty of corpsing! Because Nigel Havers has a twinkle in his eye, so I must never look at him when I act. When Griff came on to do his big speech in the cathedral, I must say, I caught Nigel's eye, and we were gone! Oh, he is a big culprit. Nigel is a naughty one. he's naughty.

This is coming out at around the same time as Murder On The Orient Express, which is full of big names. Which is going to be better?

Ours, of course! Who wants to watch that? It's going to be rubbish! Actually, it's interesting. If Johnny Depp or Daisy Ridley walk around York somewhere, no one is going to bat an eyelid because people aren't expecting to see them. I promise you, Johnny Vegas walks out, and he's just mobbed! Everywhere we go people are like, "Oh, my God - is that...?"

What's your favourite whodunit?

The Mousetrap is unbelievable. And on TV... I used to love Poirot.

Who's your favourite detective?

Oh, Colombo! Colombo was my favourite detective, by far. Just his delivery, and that actor, and the way he could make you absolutely howl, as well as go, "how does he do it?" As a kid it was unusual to like whodunits, but I absolutely used to love Colombo.