Greta Scacchi: Why did I do it? The question to ask is, if it does good, why not?

Our writer says she has always been a campaigner. But she got really worried when people started thinking she was an expert

Last week it was Clive Anderson on privatising England's forests; the week before it was Alexandra Burke on pneumonia vaccination, and the week before that, Bianca Jagger on WikiLeaks. Do celebrities make good activists? It's a question I've been exploring, having spent the past few months speaking to charities, celebrities, journalists and politicians for Radio 4.

I was interested in doing a documentary on celebrity activists, because over the years I have been quite an active campaigner myself, involved with environmental causes since my student days and, two years ago, helping to promote a documentary about the environmental impact of over-fishing. A moment in front of the camera of the photographer Rankin, wearing nothing but a very cold and slippery Icelandic cod, did the trick.

Yes, I am an actress; yes, I am writing about saving the world; and yes, I am open to the charge of getting my name in the papers because of an issue I care about. But I was – and am – rather doubtful about the way this business operates. And it all dates back to the cod picture.

There was plenty of media coverage, but it was my subsequent elevation to spokesperson for fish that took me by surprise. Within weeks I was invited to join debates on political TV programmes, to speak at the party conferences and to have talks with the minister for fisheries. Campaigners at the World Wildlife Fund and other organisations were very grateful to have a voice, and furnished me with scientific facts and figures. But I couldn't help thinking: Why me? Why not them? Surely this could have been better done by an expert.

That may be your view, too. So the question I wanted to answer as I started work on the documentary was how we actors and models and sports stars get roped in, and whether it's good that we do.

First, I should explain why we are keen to do it. I think my motivations, and those of my fellow celebrities, are the same as anyone else. If we see something that angers or concerns us, we want do something to help make it better – like people helping at their local school or going on a march or donating clothes to Save the Children. Just as everyone else gets endless envelopes and charity begging letters, so do we – in the form of requests to make appearances at fundraisers, endorse a cause or figure-head a protest. And, just like those who cannot ignore the charity envelope, some of us find it hard to say no when a small gesture can – apparently – make a big difference.

I think the point was put best by the singer John Legend when I asked him why he wanted to get involved: "To me the more relevant question is why wouldn't I get involved, because I see people suffering, I see people that need help, and I feel as if I have a position and an amount of power and resources to help me to help other people, and I feel as if I shouldn't waste them. I should do something to help other people."

Of course, there may be people who do it for the wrong reasons. There are stories of agents calling up charities after their clients have had PR problems, or looking for what amounts to a free holiday, but I thought what John had to say about that was strikingly sensible, too: "Who cares, really, at the end of the day? If the ultimate effect was that good things were happening for people that needed it, then I think it would still be worth supporting it, and maybe not nominating that celebrity for a Nobel Peace prize, but still support the cause".

Most celebrities are in it for the right reasons. Cat Deeley is a Unicef ambassador, but spent much of the past two years learning about the charity behind the scenes so that when she was promoted to fully-fledged ambassadorship, she would know what she is talking about. She told us that fame doesn't fulfil her as a person, but this activism can.

"As long as I educate myself before I go into the situation," she said, "then I can bring the bit of me that maybe experts don't have, where you know a Saturday prime-time audience of over eight million people want to hear what I've got to say purely because they know me and I've been going into their living rooms for the past 14 years."

Some celebrities have made a second career (albeit an unpaid one) out of their activism, and they typically follow a path of gradually deepening involvement: starting with fundraising, moving to publicity and ending up with lobbying.

Think Bono, Annie Lennox or Bob Geldof on development. They have become significant players, influential because governments know they are serious, know they have an audience and don't want to cross them. An interesting nugget from the recent WikiLeaks revelations came in a message from the US embassy in Rome reporting that the Italian government sustained development spending because Berlusconi didn't want to be criticised by Bono.

But an even more extraordinary story I came across was how Tony Blair and Gordon Brown used Bono and Bob Geldof as informal ambassadors to the German Chancellor Angela Merkel. They met her, campaigned widely in Germany and eventually helped to persuade her of the case for a better deal for Africa. She became the first German chancellor in many years to increase development spending.

My own political involvement wasn't at quite such an elevated level, but during the run-up to the Marine Bill in 2009 I was invited in to see the then minister for fisheries, Huw Irranca-Davies. I tracked him down again recently, and he provided me with a fascinating insight into how politics works.

He said he found our campaign very useful, because it gave him the external support he needed to get around obstacles within Whitehall. "The way to change opinion," he told me, "both the policymakers and the public, was that sort of celebrity-led campaign, which does carry more influence than if you like a minister sitting behind his desk, no matter how worthy the cause is.

"The celebrity issue helps because it sets a context in which it's easier for a minister to make a decision, knowing that he's got at least some support behind him."

That, of course, is a rather depressing commentary on our society, but it puts into plain view why many organisations are happy to work with celebrities. And they really are happy to work with us: of Britain's 30 largest charities (if you exclude charities such as the British Council and housing trusts), three quarters now have full-time celebrity managers.

The reason is simple: it's the Heineken test. We can reach places other parts of the charity cannot. When I asked one charity chief executive why people wanted to hear from celebrities such as me, he gave a very clear answer: "You shouldn't underestimate your power and ability to make very complex things simple. You're good communicators and you also have your finger on the pulse of where the public are."

Of course, we are sometimes invited in to see politicians because they are real people and want to meet celebrities, too. The former Canadian prime minister Paul Martin confirmed this, although with a twist: "I suddenly got on my schedule arranged by one of my assistants the fact that I was meeting Bono. I knew who Bono was but was not eminently familiar with everything about him. But let me tell you, my assistants were very familiar with him, and I was going to meet him, whether I wanted to or not, because they wanted to see him."

If it's easy to see why celebrities want to work with charities, it's not hard to see why the relationship works well the other way, too. And I'm pleased about that. I started research for this programme feeling a bit jaded about the value of celebrity activism. But I am already feeling more positive.

It still irritates me that we live in a media climate where a small handful of well-known people can make the headlines because they were spotted going to the supermarket or falling out of a night-club in spiky boots. But we do. However irritating you find it, there's no point trying to turn back, Canute-like, the tide of people buying celebrity books, magazines and DVDs. The phenomenon of celebrity – at least for the time being – is here to stay. And while it is, and while charities find that valuable, then many in the public eye will echo John Legend: why wouldn't we want to help?

'Celebrity Activists' goes out on BBC Radio 4 today at 1.30pm