Take care how you brush your teeth when Alistair McGowan’s around. Extravagance can send his stress levels soaring. “One of the cast members was cleaning his teeth in our dressing room and it made my toes curl,” he says, voice rising with incredulity. “He was leaving the tap on while he brushed! There’s a hosepipe ban in this area and people wonder why.”

Welcome to the world of McGowan, the superstar and obsessive eco-vangelist. Showbiz and high-profile profligacy are normally close as clones, but television’s prime impersonator arrives for our interview riding a bicycle that looks as if it has been pulled out of a skip. He’s hardly dressed by success, either. An old black T-shirt and jeans frame his tall, fit frame. He’s complaining about careless, bike-blind drivers.

The gentle foil of famous fops such as David Beckham and Robert Kilroy-Silk is taking time out from starring in Gogol’s The Government Inspector, at Chichester’s Festival Theatre, to promote a charity initiative: the London Tree-Athlon, a race that aims to fund tree-planting schemes for inner cities. But as we settle in the fusty mess of his dressing room, he is keen to show how eco-activism should start at the smallest scale.

“There’s now a recycling bin in here and there’s one out in the theatre,” he says. “They always leave the lights on in here. And the fan. I make sure they’re switched off. Before I came, no one thought about this stuff. It only takes one person to make changes.”

McGowan notices my raised eyebrows. He must be used to this. “I’m obsessive as a person but I don’t think I’m freaky,” he says. “This is simply about changing your behaviour slightly. I would never use a one-piece disposable razor, I use ones where you can change the blade. Extreme environmental campaigners would not shave at all. I’m just contributing less to landfill.”

Early in his career, years before his stint doing voices on Spitting Image helped to lift him to stardom, McGowan appeared in a TV ad for gas-guzzling cars. Now he hates four-wheelers, won’t drive and lists Jeremy Clarkson as his public enemy No 1. He won’t even say what carmaker he was advertising. “I was a struggling actor. It’s the thing that I am most ashamed of.”

So what prompted his Pauline conversion? The answer, he says, lies in his theory of people, a theory that underpins his remarkable talent as a mimic. “I believe in the human soul, and I believe that it is like a mixing desk; we all have the same channels but the faders are at different settings. Impressions are about finding what levels these faders are set at. Kevin Keegan has passion at 10, success at 9 and ambition at 10.”

How do you divine someone’s levels? Usually, it’s all in the voice, he says. “It gives away huge amounts of personality. With Beckham, I thought straight away, early in his career, ‘What a shy person’. His voice is at the back of the mouth. He’s too self-conscious, the pitch is too high, there’s a quaver, the mouth is closed at one side; as a speaker his confidence is set very low. As he emerged as a personality, he turned out just like that, though he’s highly confident as a footballer.”

Halfway through the explanation, McGowan’s voice mutates into Beckham’s thin Essex lisp. Then, in a room-filling boom, he adds: “Clarkson, on the other hand, has a deep, assured voice. It suggests someone in touch with their person, though overconfidence is just as unattractive.

“He is the antithesis of all that I hold most dear,” McGowan adds in his own reflective timbre. “I tried to do him on the Big Impression, but it didn’t work because it was so full of bile. Generally, to do the people I do, I have to want to get inside their heads. With Clarkson, I don’t want to know.”

As a professional shape-shifter, McGowan, 40, is fascinated by how people’s inner settings can change. “Age or experience can do it. I know: 17 years ago, one little piece of information shifted my environmental-awareness level from 1 to 10 in one go. I had never thought about it before, but then I read a Sundaynewspaper article about the way we create needless waste. From then on, I thought about how everything in nature is cyclical; how things are born, die and are re-used. We ignore all that for the sake of consumerism. I am baffled by the amount of things people buy because they believe they will enrich their character or status. I think it’s all about what’s in your self, but instead people are continually shopping.”

He is a member of the Green Party and a supporter of Greenpeace but admits: “We all have our weaknesses. Clothes are mine, though I am very good at recycling them and I don’t wash them that often (he laughs maniacally).

“We overwash. I especially never use a tumble-dryer. What scares people about any environmental talk is that it sounds all about sacrifice, about giving up things and status. The alternative is a simpler way of life and a greater amount of happiness.”