Jess, at seventeen, is more naïve adult than typical teenager. She has been brought up to attend church and learn exorcism from her mum, and most of her interaction is with her friend Lynsey, who I don't imagine to provide her with much intellectual stimulation. Jess has a lot of confidence in her beliefs and, although she certainly isn't part of the popular crowd at her all-girls school, she would be too self-assured to be have been a victim of bullying. She is self-contained, intelligent and perhaps a little bit lonely when Giving up the Ghost begins. For Jess, the story we share with her is a coming of age and her initiation into the adult world.

Dean is probably the first boy of her age who Jess has interacted with without her mum watching from the other side of the room. He is certainly the first to have taken an open romantic interest in her and asked her out. There is no doubt that Jess fancies him back, but for the first time in her life she is forced to make a serious decision about her confidently held views. Either she must acknowledge that Dean is something sent from Satan, or that the facts of the world are far less clear cut than she thought they were. The end of the play doesn't see her rejecting her beliefs – perhaps she doesn't, entirely – but she will never stop questioning again.

I like to think that Jess's relationship with her mother is a very real one, combining the bond generated through being a tight team of two for seventeen years with the struggle as Jess begins to crave a chance to become her own person. And with Claudette's daughter Alice playing Jess, and Mary-Jo's daughter Amelia understudying the part, a lot of my work was already done for me. 

Mary was definitely the most challenging character to write, as the word 'exorcist' in itself attracts a whole host of mostly negative connotations. I didn't want her to be a hellfire and brimstone preaching, right wing stereotype, and I definitely didn't want her to be the villain of the piece, especially with Carrie developing into a more sympathetic character than previously planned. At first glance, Mary is certainly a conservative Christian, a strict mother and a harsh judge of any perceived moral deviance. But as with Dean's rock star bravado, this is very much a front. And although Dean is in many ways her polar opposite, as we get to know Mary we realise that her story in fact has many parallels with his.

We also see Mary's softer side in her relationship with Jess and, occasionally, with Carrie. Having lost her mother at a young age, she is constantly motivated by a desire to keep her daughter and younger sister safe, ostensibly from the corrupting influence of Dean, but ultimately from anything which falls outside the system of beliefs with which she cushions herself. On top of her similarities to Dean, Mary's character is made more paradoxical when we find out, despite her insistence that Jess stay away from boys, that she had her daughter at seventeen. In reality, this was written in to fit the ages of the actors – but as with Carrie/ Claudette's pregnancy, it seemed to add to rather than detract from the story.

If you think Mary's portrayal is especially convincing when you see Giving up the Ghost,you might be interested to know that actor Mary-Jo once wanted to be a nun, and brought her daughters up for many years as a single mum – just like Mary did.

As a foil for Carrie, the first thing Dean had to be was dead. And being dead, the next thing to find out was how, when, where he had died. I can't remember deciding that he should be from the nineties, but I think it was an early decision - it's a decade I remember well, I like nineties music and hate nineties fashion, and of course Mary Jo's memories of being Dean's age at the time would come in hugely useful. 

As for the way in which Dean died, suicide was always going to be the most interesting option dramatically. Murder would make the story into a crime or revenge drama, illness or accident didn't give the amount of dramatic scope I wanted for the length of the play. Making Dean suicidal meant that his journey would be an internal one, his battle would be with himself as well as with the characters and attitudes of the world he finds himself in. And of course, suicide is something that is frighteningly common in young men, and something we still don't talk about enough. At the time of writing, two young men who were in my year at school have committed suicide; I have met others of my age who have tried. Whilst I didn't want Dean's suicide to overshadow the rest of his character, it wasn't something that could be skirted around either. And with so many negative media depictions of the mentally ill, it seemed important that Dean would be the one character not to be a bit of an outcast - unlike the other three, he is generally easygoing, charismatic, the one we'd all want to have a pint with; but also the one who experienced despair so deep that he ended his own life.

The other side of Dean's story is his relationship with Jess - though this is arguably more life-changing for her than it is for him. This is where we see the man Dean was in happier times and, in true love story fashion, Jess becomes his main reason to keep living - or trying to live, as the case may be. When Giving up the Ghost was first workshopped in front of an audience in Glasgow, the question of whether Jess and Dean's love should triumph caused quite a debate - but you'll need to buy a ticket to find out if they get their happy ending. 

Carrie was the first character to take shape in Giving up the Ghost, and was definitely a direct response to a certain famous television psychic who regularly tours to the Glasgow Royal Concert Hall, where Claudette and I work as front of house staff. I shan't mention her name here, and unless you are as uncomfortably familiar with her work as we are, you probably won't recognise her, as we did reluctantly agree that Claudette should tone down her very accurate impression in performance as Carrie. Suffice to say, the psychic in question peddles a brand of emotional manipulation that seems to target the vulnerable and make assumptions about their loved ones which - if believed - would change the way they were remembered. One particularly unpleasant 'reading' which sticks in my mind involved the psychic advising grieving parents that the roots of their son's suicide lay in a struggle with his sexuality - something which the parents admitted had never crossed their minds previously.  

It seemed then that Carrie would be a fairly despicable character, although what really attracted me to writing her was the potential for humour in her incompetence - the opening scene of Giving up the Ghost is all about this. The character of Dean was initially envisaged as a way to mock Carrie further, the ultimate irony for an utter fraud being to have their lies come true. But as the two characters began to talk, Carrie developed into a deeper character than the callous and calculating mercenary who stood on a stage and lied to desperate people. As soon as I introduced Mary and concluded that they were sisters, Carrie's whole back story materialised. Both Carrie's and Mary's views are extreme and potentially alienating to the average audience member - but as intense reactions to a childhood trauma, they become both more understandable and more compelling. 

Entirely unintentionally, mothers and our relationships with them are a major theme in Giving up the Ghost, so the fact that Claudette will be six months pregnant when we perform at the Fringe only adds new layers to Carrie's character - you might almost imagine it was intentional.