At 16 Charlie is just starting high school, but while the friendship and partying seems to come naturally to everyone else, he can't help feeling like he's always on the outside looking in. Sick of feeling so alone, he decides to see what life is like from the dance floor and after accidentally befriending the charming, loud-mouthed Patrick, he comes to realise that being an outsider doesn't always mean being alone.

But life isn't always that simple and after falling head over heels for Patrick's half-sister, Sam, what follows is Charlie's attempt to navigate the minefield of sex, drugs, first dates and mix tapes, while coming to terms with a dark secret of his own.

With the DVD of the Perks of Being A Wallflower out now, we phoned up author and the film's director, Stephen Chbosky, to talk teen angst, sequels, making movies and of course what it was like working with Emma Watson, Logan Lerman and Ezra Miller.

To get straight in there, making a movie out of your book and being so involved with the process must have been a dream come true. How did you feel when you got the call that it was all going ahead?

It really didn't happen like that. It all started with Emma Watson, who had lots of meetings with heads to studios here in Hollywood. She was our champion, she really wanted to make it happen and it all started with her.

So you'd seen Emma as someone to play Sam from the beginning?

Yes definitely. It was when I saw her in Harry Potter and the Halfblood Prince actually. It was the scene where she was with Harry outside the common room and she's crying and it just broke my heart in that scene. I just thought she was fantastic and I noticed that every time I saw her in a movie, she just got better and better, and that was the first clue.

What brought it home to me was when I met her in New York in person, and I realised that I was fundamentally interested in this girl and she was fundamentally interested in me and my project. I knew how much she had to prove to herself beyond Hermione, that she could do all these other things and I knew that she needed to believe it, that she had the confidence to do it and I just had it. Watching her, I just knew.

Was it a similar feeling with Logan and Ezra or did they come along a lot later in the process?

With Logan and Ezra it was slightly different because they both auditioned whereas Emma didn't, but I still had a feeling about them when I cast them.

Do you think it was something you could have ever trusted anybody else to direct?

I don't think I could have, it was something I wanted to do myself. I was either going to direct the movie or it just wasn't going to happen. The book was doing fine and I just didn't feel it had to be done and I couldn't let down the fans. Some things aren't worth it and I had a very particular idea of how it should be.

So going right back to when you sat down and started writing, where did the story begin?

It was a combination of ideas I'd had really. I'd thought about writing the book for a long time and these images came to me over the years – the tunnel and the friendships and the songs. Parallel to this, I thought of this idea that I called 'Letters to a friend' and I was going to write a novel from the point of view of a young person and it was going to involve someone finding them in a phonebook in 20 years. So one day, after a break up, I just sat down and put the ideas together because I realised that Charlie was the boy writing these letters and in one moment, the ideas came together and I never looked back!

One thing we did notice about the film is that the tunnel song is different to the one in the book! Why did that happen?

You're right, we did change it. Well, when it came to actually flying through the tunnel, when you saw it, Landslide just wasn't energetic enough. To me, this is a triumphant moment, not a sad, somber one.

Did you try it with Landslide first?

We tried it with a lot of songs, but to me Heroes was just perfect. It's so terrific, so cathartic when Emma is up there flying in the truck. It just seemed to fit.

The music is clearly such a big theme in the book and Emma said that during filming she and Ezra formed a band with Logan, and he wrote them songs. Did you get to hear them?

I did. well, I heard them a little bit. I actually joined them a bit as I play guitar and I sing a little bit, so there was one time we all got together and played. Logan plays the piano, Emma sings and they let me join in, they were really living it being together. I mean, these guys, these young actors have so much pressure on them and on their lives. I can't imagine being 12 years old and having a multi-million dollar production weighing on my shoulders so this was a chance to cut loose.

It seems like they were a really tightly-knit group. So the friendship spilt over into real life?

Definitely. They are all lovely. You have to understand that when I was casting the movie, I was casting people, and it was very important to me that the people that were telling the story were kind and supportive and were nurturing of each other - and of me! Because I didn't want divas, that's not how I deal with things.

Not only were they good people, but each of them - for different reasons - had something to prove with this movie you know, and that's exciting. If you've got people on board that are hungry to prove something, there's nothing better, there really isn't. I look at it this way, like the way I look at rock bands. For the first couple of albums, they're on fire. They have so much to say and do, and that's how it is for them.

Well we definitely couldn't have thought of better people to play Charlie, Sam and Patrick – they are all individually so like how we imagined the characters in the book. If you were ever going to take one of their characters any further, whose would it be?

Well I'd like to write about all the characters but it would have to be Sam AND Charlie. You know, in different stages of their life, like working in an office or whatever. There are always certain people that stick out for you and ultimately the love story between Sam and Charlie is something that will always be there for me. I think it's the enduring story, them together.

Do you think it's something you might ever go back to?

I don't know, I might. I love the kids and it would be lovely to make another film with them. Maybe we should just get them all back together in ten years and tell another tale!

We'd love that and at least Charlie will have left high school long behind by then. He doesn't have the best time to start with, but what were you like in high school?

Well I had my public self and my private self, which were two very different people. I mean I had friends and I was an athlete – I played soccer – and I did musicals and that kind of thing right through school too life was OK and externally I was great. But internally I had a lot of questions and a lot of demons, and so some of those came out with Perks.

So is it Charlie you identify with most or did you put different parts of yourself into the different characters?

I relate to them all but of all the characters, Charlie is definitely the one closest to my heart. It's kind of a combination between Charlie and then externally, the smart-ass part of Patrick.

We slightly fell in love with Patrick, he's definitely one of our favourite characters. If you had a time machine and could go back and give your half-Charlie, half-Patrick teenage self some advice, what would you say?

The advice I would give would probably be to tell yourself and the people that you trust who you really are and you'll be accepted for exactly that. I think kids are very like chameleons, partly because they're trying to find their own identity and partly because they don't know if they can or should admit all these things that are going on inside of them. But generally I think you won't be happy until you admit things to yourself and to the people you're close to.

That's kind of the message of the book too isn't it?

I could say things like 'you are not alone' and 'you'll get through', and talk about some of the issues in the book and how people get through, but that's exactly it. That's what Perks is all about!

The Perks of Being a Wallflower is out on DVD now. It's awesome and you should probably get it. And the book.