Winsome Pinnock has always written from conviction and will take on political issues not because she wants to shock or provoke but because that is the kind of writing that interests her. Inevitably, a play about a racial killing and a conspiracy to prevent the murderers from being sentenced brings to mind the death of Stephen Lawrence, but Winsome Pinnock based her play on a different incident.

Can You Keep a Secret? is based on the killing of an Asian man a few years ago. The girlfriend of one of the killers found out about it and her dilemma was whether or not to tell the police. If she did, she knew she would be ostracised. She was an ordinary young Londoner and I thought it was incredibly brave of her to give evidence knowing that, in her community, that was something you simply did not do. I was very struck by her heroism and it's been on my mind for a long time.

Can You Keep a Secret? opens with the casual meeting of Kate and Aleysha followed by a fight between Derek and Sean. Without doubt Derek is provocative. He taunts Sean, who is unable to cope verbally and resorts to the only strategy he knows, beating his opponent into silence. Sean is undoubtedly out of control and Derek appears to be asking for trouble. The language here is shocking, the violence explicit, the emotions raw. All except Weirdboy, an outsider and an observer, are drawn into complicity to protect Sean.

This is raw emotion but I think young people will handle it because they are honest. The language is not provocative or inflammatory. It is the language of young people, the language I hear as I am walking in the street. The language is at times racist, foul and violently homophobic but it is the language of the characters I've created. You don't clean up the language because the play is coming to the National Theatre. Some people will find the killing of a black youth on stage shocking. When I am writing there are conscious choices and instinctive ones, on one level thinking of the best possible ways of telling the story and then there is an instinctive level. I didn't feel it was right for his death to happen offstage where the audience could either imagine it or choose to block it out. Some things are unimaginable and this is one of them. In writing I had to confront the death and other people have to face it as well.

Derek is young and intelligent. He is not a tight-lipped person who will walk away from trouble. He is volatile and has his own prejudices. He is brave but like any young person he does not contemplate his mortality or envisage any fatal consequences.

Derek is much more articulate than Sean and he allows his clever exuberance to get out of hand. He manipulates Sean and pushes him over the edge. He goads him in the way that young people do when they're having an argument and want to win. He doesn't know what the consequences are. He simply defends himself. You can relate this character to real life but you have to remember that in the end he is a character in the play.

Kate is in love with Sean, she thinks that she needs him and will keep silent to protect him, but there is a part of her which knows that it is wrong to keep silent. Although she promises to destroy Sean’s bloodstained jacket, she keeps it and later gives it to the police. At one point Derek reappears in the play to talk to Kate.

Some people will see Derek as the personification of Kate's guilt and grief but for me he is real. In the world of the play you can believe that he comes back and pleads for someone to speak on his behalf. With cases like this in the real world the victim disappears and his side of things ends. Somehow there is something to be reclaimed. There are deep myths and stories about the dead returning to seek justice and the theatre is the place where you can explore these ideas.

Winsome Pinnock sees Can You Keep a Secret? as being about friendship, loyalty and making a stand. Looked at as a whole, the society portrayed in this play is dark and depressing. A young black man is killed, the murderer’s friends conspire to protect him, the police suppress evidence and the community closes ranks, allowing Sean to get off. Looked at from Kate's point of view, the play is optimistic. She moves from seeing the world in one way to a radically different view. She comes close to a dark experience and is able to do something very positive and to redefine her place in the world.

Kate loves Sean and for this reason is willing to keep his secret. But there is an underlying conflict within her and to a lesser extent within the other characters as well. Deep in her heart she knows what the right thing to do is but at first she is unable to act because of her loyalty to Sean and his friends. She and Aleysha have little in common but they come together at the end and Kate is drawn closer to Aleysha and Derek’s world.

Winsome Pinnock is convinced that drama can affect people in the way that she has been changed by seeing and reading plays.

I have had some real emotional experiences in the theatre and if drama can affect me it can affect others. It is not a passionless medium. It is one of the last areas where you can have a public debate. It's good if a play invites young people to ask questions. For me this play says Derek’s life is not something that was insignificant but it is worth everything to his family and they will not let it rest until justice has been served.