My metric for measuring someone’s likability is this: would you mind being stuck in a lift with them? Elizabeth Morris: confirmed as someone you would absolutely not mind being stuck in a lift with.

We catch up on a Wednesday afternoon. She’s filming an interview for London Live which she kindly lets me tag along to. We chat in the green room. I ask her where she lives (bit creepy, Beth) and she tells me. I mention that I have an ex-boyfriend who lives there. We start talking about other exes, and the unique frustrations of dating in London. She’s deleted all her dating apps, and I’m considering doing the same. I’m sitting across from a world-building, creative powerhouse who’s written dialogue for Gary Oldman and is currently working on her first solo feature script and a children’s book and I’m asking her about… boys? Luckily she’s called to the interview before I can have her braid my hair or ask her what lipstick she’s wearing (just kidding, it’s an iconic Smash Box red).

The interview isn’t live, so I expect a lot of fluffing and accidental swearing and having to repeat. There’s none of that, though. She’s confident, easy to watch, dynamic. I lean over and quietly ask her publicist “How many of these has she done?” “Not many.” Oh, so she’s just good. Cool.

She’s here to talk Killers Anonymous, a crime thriller starring Morris alongside Gary Oldman, Jessica Alba, Tommy Flanagan and Suki Waterhouse to really name but a few (there’s bloody loads of famous people in it and I’ve got a word limit). She co-wrote the film with director Martin Owen.

The concept for Killers Anonymous has its genesis in an earlier film that Owen and Morris collaborated on, L.A. Slasher, a satirical horror-comedy about the artifice of Hollywood. The film’s killer, just after doing some killing, heads along to a support group to discuss his murderous urges with other Bay Area slashers. The scene is less than two minutes long, but it’s a concept that her and Owen both wanted to revisit. Six years after they wrote the film, Morris gets a call. There’s been interest in the idea, and Owen wants to know if she’ll come back on board. Spoiler: she got back on board.

Her personality – earnest, creatively energised, ambitious – seems to be the product of persistent hard work and trusting her instincts

She plays Krystal, a butterfly-knife-wielding babe who she describes as “an intimidating, foul-mouthed, overbearing, wild-card... a really nice girl.” She’s joking, but I’d definitely go for a pint with Krystal based on that description alone. She worked with stunt and fight coordinator Abbi Collins to master use of the knife, as well as to co-ordinate the film’s many fight scenes. Perhaps inspired by this, she recently started boxing, which she tells me is great for letting out aggression. “I’ve been training with this amazing woman called Sam Schnitzler, who’s an actress who does a lot of fight stuff in films. We started talking and she said ‘do you want to come and train with me?’ and I thought ‘she’s going to kill me’. She’s so tough, she’s so badass.”

Morris herself seems plenty badass, both on screen or off, both with a knife in her hand or a cappuccino. She’s interesting and alert. She’s cool, and quick to laugh. If I was a male interviewer this is when I might mention how beautiful she is, and spend a whole paragraph describing her jawline or skin tone like a serial killer. Luckily I’m not a pervert, so we’ll skip that. How she looks is an accident of genetics. But who she is – earnest, creatively energised, ambitious – seem to be the product of years of persistent hard work and trusting her instincts.

Though born and raised in Gloucestershire, she’s at home in London. “I’ve been here since 2011. I love it. My whole family originate here, my sister’s here, so it feels like the natural place to be.” Her family home in Gloucestershire functions as an escape, a place to go when she’s not needed in London and wants to spend time with family. “I love home. It’s in the Cotswolds so it’s all trees and fresh air. It can be a relief to go back. But then I always miss London and I’m ready to get back to the buzz of it.”

You studied musical theatre and performing arts at the University of Gloucestershire. Why not London?

I did my A levels, then left school when I was 18 and had intended to apply for drama school. But at the time my grandmother was really unwell so I ended up being a full time carer for her. And it was the first year that this course had opened, and it was on my doorstep. We had teachers coming from The RSC, from Doreen Bird, from the West End to teach us, so we got this really good quality of teaching. I did that for 2 years and then moved straight to London to start work.

And did you always want to live this kind of creative life?

Always. As long as I can remember I’ve always performed. As a kid I would be in the centre of the room making people laugh, putting on plays and shows. My childhood was going to my friend’s houses, and us going out into fields to put on plays and do dance routines and sing at the top of our lungs on peaks and hills. I was always writing, always dancing always singing always acting, always drawing. As a kid I was always so set on doing something creative.

So kid Elizabeth would be pretty pleased with how you’ve turned out?

To be honest, I think she’d be pretty chuffed. Though I think she’d be very surprised that I ended up working in film. It was always ‘stage, stage, stage’. I actually don’t think film ever really entered my mind because it felt so out of reach. I don’t think I ever thought that’s what I’d end up doing, but I think she’d be pleased.

Let’s talk Killer’s Anonymous. Is there any scene you’re really excited for audiences to see?

For me as an actress, there’s a fight scene I have with Jessica Alba. There’s guns, there’s kicks, there’s punches, there’s flying off tables.

So normal day at the office, then?

Absolutely. That I think, teenage me would have been like “YOU’RE KIDDING. WHAT.” It’s a really fun scene. And I also sing in it, which was something that was really scary to do.

Hang on, but you’re trained in musical theatre and wanted to sing on stage?

Hundred percent, but I’ve always had a real anxiety around singing, so then suddenly to be singing in Jessica Alba’s face… But that scene is a lot of fun. It’s near the beginning of the film.

I’ll watch out for it. So what about as a writer? What scene stands out?

There’s a scene between Gary Oldman and Suki Waterhouse which was a really fun bit of dialogue to write and also then to see them both deliver. They’re discussing the urges to kill and Gary’s character is trying to talk Suki’s character off a ledge so to speak. He’s trying to discourage her from making her next kill.

You wrapped filming about a year ago, what have you been up to since then?

When production stops, you can go into this lull where you think ‘what next?’. And if you haven’t got something lined up, in particular if you were a writer and an actor on that project, it can be all consuming. You haven’t really thought about anything else, and then it stops and it’s like ‘oh god what now’?

So I went away and thought ‘I want to write my own script about topics that are really, really important to me.’ I wanted to talk being single, online dating, mental health, the drinking culture in Britain, which is something which I think affects so many people. It was something that I wanted to write about, and so I did.

And that became “We’re All A Bit Like Lilly”. What else can you tell us about that project?

We’re All a Bit Like Lilly is a dark comedy drama. It’s the first feature film that I’ve written by myself. It’s also my original idea. I have Catherine Gray on-board as script consultant.

How did that collaboration with Catherine Gray come about?

So at the same time as working on this script I was reading a lot. Books by Dolly Alderton, Bryony Gordon. I was also listening to Elizabeth Gilbert and Brene Brown, so just all these incredible authors and speakers and influential women. And there was this one book in particular called the Unexpected Joy of Being Single by Catherine Gray.

It’s an autobiographical book, and I have so much admiration for that, because when you write fiction you can be a bit anonymous about it. But she’s gone ‘This is my experience, this is how I’ve dealt with it.’ So I just got in touch with her and said ‘first of all, thank you for writing this book. You’ve helped me understand a lot of things in my own mind. Is this project that I’m working on something that you’d be interested in collaborating on?’ And she said let me read it – so I sent her my stuff.

Did you ever hesitate or think, ‘I really admire this person. There’s no way I can send her my work’?

I immediately though “oh my God. She’s a Sunday times bestselling author. What have you done? You idiot. Why have you done this?” But she got back to me, and said she really enjoyed it and that it something that she would be interested in coming on board with.

How did you research this script?

Years ago I read this interview with Kristin Wiig, and she said her process for writing Bridesmaids was to get her female friends together and discuss all the ridiculous stuff that they’ve done. And I thought ‘well my friends are a bunch of ridiculous idiots’. So it was ‘right, girls. Come on over. Let’s reminisce.’ I also got on Instagram and said ‘Tell me your stories, ladies! What embarrassing things have you done when you’re drunk, what are you ashamed of, tell me about your dating experiences.’

So fans of Dolly Alderton and Bryony Gordon and Catherine Grey should be looking forward to this?

Yeah, 100%. I hope so, at least.

You’ve worked on a lot of really dark films. What frightens you?

My first experience with genuine fear was actually film related. I had a babysitter and she bought Stephen King’s ‘It’ over on video. I vividly remember seeing this video case on the dining room table and it had a clown on it, and at that age I didn’t make the connection that it was a scary clown. It was just a clown. And I begged and begged and begged her to let me watch it. Anyway I must have been really annoying because she eventually just let me. And it started off this immense fear of clowns. Really intense.

[Side-bar: calling her fear of clowns intense is possibly the greatest accidental joke I’ve heard in some time. Intense. In. Tents. Amazing.]

Are you still scared of them now?

I think I’ve managed to kind of curb it. Although do you remember that thing recently with Killer clowns, when they were just going around. I don’t think I left the house for weeks. But then I also have a really morbid fascination with them as well. Same with sharks. I got this intense fear of them, because of Jaws. But then I’m fascinated with them.

So another movie fear, then?

Yes. God, maybe I’m just scared of films.

Do you ever feel underestimated in this industry?

There was a time when I was about 22, and I had co-written this script and I working in a team with a lot of older men, and this producer just didn’t believe me that I had written even a single word. We were going through the credits, saying ‘ok who are we crediting for the writing’, and my name wasn’t on there. And I said ‘hang on, I literally wrote half of that script’. And it was like ‘Well, prove it:’ So I did.

I went into my laptop and I had to dig out the first draft. I had to literally find evidence of the fact that I’d written even a word of it. That experience taught me ‘okay, put your foot down, make sure you’re vocal, don’t let anybody assume that you haven’t been as involved as you have.’ I’m getting there with saying ‘I did that, too. It wasn’t just everybody else.’

And when you’re knocked down like that, how do you get back up?

This is definitely something that I’ve had to learn how to do along the way. When you have doors shut in your face, or someone says no, I think the most important thing you can do is set yourself goals and just work towards them. Even if they’re tiny goals, just keep going. Because once you stop, it’s so easy to get into a spiral, and then things just slow down, and then you go into self-doubt.

I think you have to re-evaluate what you’ve already done, and think, “well okay, you’ve gotten that far.” And then you set a goal for what you want to do next, and just don’t stop until you get there.

Killers Anonymous is in cinemas now