Ms Lewis is the woman responsible for overseeing Oxfam’s campaigns with celebrities and describes what must have been the photoshoot from hell for the Oscar winning actor Firth.
“Well, we heated the first vat,” she explained. “But the problem with coffee when you’re dumping that much over someone’s head, it just gets colder and colder. By the end it couldn’t have been nice.”
It was a nice idea to boost an Oxfam campaign for free trade and the King’s Speech star still managed to look cool in every sense. But Ms Lewis didn’t stop there, she covered REM’s Michael Stipe in milk. “His hotel room ended up smelling of baby poo,” she laughed.
“Oh my God, there was milk everywhere. It was a plush hotel. I don’t think they knew what we were planning. We just went in and covered the floor with plastic.”
And when dumping chocolate over the head of Thom York had the unforeseen effect of making his eyes sting, the mean and moody Radiohead man was sweetness itself.
For the stars trust Ms Lewis, who started working with Oxfam as a bookshop volunteer before going on to become the charity’s international artist liaison manager.
Whether it is a case of exposing themselves to uncomfortable photo-shoot indignities for the latest campaign she has dreamt up, or being escorted around vast refugee camps in Africa, they know their fame will never be put to better use.
There is of course nothing new in charities enlisting famous people to champion their causes.
Back in the 1950s Marilyn Monroe visited US orphanages, The Beatles famously donated to Oxfam in the 1960s and actress Julie Christie campaigned on Cambodia in the 1970s.
But in this celebrity obsessed age, Ms Lewis, 43, ensures Oxfam benefits to the maximum from its famous friends and supporters.
“People do not want to see famous white people holding starving babies,” said Ms Lewis. “Oxfam has moved away from that kind of thing and I don’t believe it washes with the general public anyway. We want to see the situation where people tell their own stories but these stories are amplified by the celebrities.”
The sufferings of a woman sitting in a refugee camp in Uganda has next to no chance of ever getting her story heard – but if it is told to a Hollywood star, who then recounts it to camera, the story will be heard by millions. It really is that simple.”
“What we want is someone who knows the organisation and who can speak in a creative way, with integrity about Oxfam. If you are putting them up to speak on our behalf, they need to know what they are talking about.”
She used to tease Colin Firth that he could now do the “Oxfamology” exam, as he read almost Oxfam’s policy documents to understand the issues. They are currently working together on a new secret project to mark the Oxford based charity’s 70th anniversary.
Ms Lewis’s own time with Oxfam goes back 25 years, when, after working as a volunteer in London, she agreed to stand in for a shop manager, who was ill. She arrived in Oxford 12 years ago and now lives in Fyfield, with husband Marcus, and their daughters Lula,13, and Freya, 10.
The impact celebrities can make as speakers never fails to impress her. She recalls last December seeing the Bollywood star Rahul Bose reduce an audience to tears, before overseeing an auction that raised $1m. “These are people who can hold an audience and are so eloquent. No one from Oxfam could have done that.”
For the stars, such involvement inevitably brings accusations of self promotion and the risk of derision for their do-gooding attempts. “The people I work with hardly need to build their profiles. They are already at the top of the tree,” said Ms Lewis.
“It’s like one of them once told me, which is better, to tell the grandchildren you used celebrity to make a difference, or get a table in a restaurant?”