Caroline Paterson is wound up and wired when she arrives at the Glasgow Maggie's Centre. Production is under way on her first film, which is set in her home city, and as its co-director she's feeling the strain. But she's desperate to take time out to view the centre because she's heard so much about it from her friend Laura Lee, who happens to be the chief executive of Maggie's. Lee lives near Paterson in London, and their sons are in the same class at school in Hammersmith. It's Paterson's first visit, and as she looks round the cosy and welcoming space she emits a long sigh. "I had no idea it was so lovely," she says, visibly relaxing.

Although the Pollok-born actor/director has no personal experience of cancer, she is an active and vocal supporter of Maggie's, the UK-wide cancer caring centres. "I would really want to come here," she says. "I love the building - so much thought has been put into how it looks. It acknowledges that we all feel great in nice surroundings and a welcoming atmosphere. But what's also great is the support it offers, the telling of the small things you want to know about and want to ask. This is a place where there's lots of things going on, and I'm sure that helps."

Paterson's appreciation of the caring ethos of Maggie's reflects her own concern for people who need a voice, and her determination to cut through social prejudice. She became a household name through her appearances in TV soap operas, first as Ruth Fowler in BBC1's EastEnders, subsequently in River City and most recently as DC Weller in Coronation Street, but she is most committed to directing theatre and now film.

Her new project, Wasted, attempts to uncover the truth about Scotland's neglected underclass by telling the story of two young homeless people. She co-wrote and is co-directing the film with her long-time associate Stuart Davids, with whom in 1990 she co-founded the theatre company Raindog along with Alexander Morton and her former boyfriend, the actor Robert Carlyle. Raindog first performed Wasted as a stage play 10 years ago, starring Carlyle. Paterson believes the subject of displacement and homelessness has become even more pertinent in the intervening decade.

"It's a state-of-the-nation piece," she says. "It's about why we continue to choose to ignore the kids on our streets, and it's about their lives. I find it frightening that we don't encourage or support them. Society looks at youths as no-gooders and thugs, and newspapers only encourage our terror of them. But what happens when we walk away? What is their life really like? They are still human beings, after all.

"I thought things would have changed for the better since Thatcher, but there are even more vulnerable young people out there now. Parents need to go out and work, but that means there's a diminished sense of community and a feeling that people are not really being looked after."

Does her film intend to seek answers? "The displacement of young people is a complex issue, but we should be finding ways of bringing our children back. This is our society, whether we like it or not. Some of Wasted will be shocking and some of it will be uplifting. I just want people to take a second look at what we're doing to young people."

It's not all doom and gloom. Wasted is also a love story between two young people, played by Glasgow actors Neil Leiper and Emma Hartley, who are relative unknowns compared to their co-stars Kate Dickie, David Hayman and Gary Lewis.

"To me it was important to not know these two kids," explains Paterson, "which made it difficult when looking for extra funding. Private financers want the big names."

Making a film is something Paterson says she has always wanted to do, but finding financial backing took a year and was "like climbing Everest". With the help of her friend, mentor and executive producer David Hayman - who gave her one of her first breaks at the 7:84 theatre company - she's managed to get £320,000 from Scottish Screen and the Glasgow Film Fund.

"It's not a huge amount, but I can just about make a film with that," she says. She aims to have a rough cut by September, then will raise more money to get a print. Distributors are already in place. She is still on the lookout for private funding.

Scottish Screen's £150,000 is from its Express Film Fund, which aims to fast-track film production. For Wasted this means five weeks' rehearsal and a four-week shoot. "Having that rehearsal time is unusual and I want to improvise it. I want to see if it's possible to create a whole film in a room and then shoot it in four weeks. It's a very, very short time. In many ways it's an experiment because we're trying to find a different way of film-making. There will be lots of headbanging and trying to find ways of making it work. The pressure is enormous. But thankfully, Kate, David and Gary are fantastic improvisers."

Paterson, 42, is an ardent fan of the collaborative approach being adopted by the National Theatre of Scotland, and says she would love to do more theatre work. Her most recent appearance was as the lead in a touring production of Liz Lochhead's Perfect Days.

She lives in Shepherd's Bush in London with her partner Claudio, a hairdresser, and son Louis, 10, and daughter Ruby, six. She's already missing them (Ruby, she says, looks very Italian while Louis is more "Scottish" looking) and will bring them up to Glasgow during the school summer holidays because a whole summer is "too long for me to go without having a bit of them". In the meantime, Claudio looks after them in the evenings, while friends such as Laura Lee help with the school run, childminding and football duties (their boys are in the same team).

When the two women first met there was an instant connection, Paterson says, and they have been firm friends ever since. "Laura often helps me out when I'm away," she says. "I am gobsmacked by what she did for Maggie Jencks (the founder of Maggie's Centres) as her nurse, and for building up the empire of the centres so self-effacingly modestly. She really is a wonderful woman."

Wasted will be shot entirely in Glasgow, using locations that show the city at its best. When we meet, Paterson is still scouting for a place for the characters to live. "I'm looking for a run-down building but with beauty in it as well," she says. "I don't want to use the usual druggie, downbeat sink estates; I want Glasgow to look its best. We'll be using west-end parks, streets, fountains and back alleys. I love Glasgow's back alleys. They're very like New York."

Research was facilitated by shadowing the Barnardo's Street Team, Shelter and the Glasgow City Mission. "I've been on the street. It's very hard for working girls and boys who only do what they do for the money. It's hard for them to get out of that life. Hostels are supposed to be temporary but some people have been living like that for years and years."

She feels inspired, rather than dispirited, by the people she has met in preparing the film. "They are the most resilient, wonderful human beings," she enthuses. "I honestly don't know if I could do what they do. They've changed my views on humanity. I do believe that people are basically good, and that there's no such thing as an evil person.

"But so many people out there need a lot of love and understanding."