Filippou with Angeliki Papoulia in Alps
The 41-year-old explains: "I used to work in an advertising company for 10 years as a copywriter and that's where I met Yorgos Lanthimos. it wasn't like it was the dream of life to be a screenwriter. He proposed in 2000 to work together on a screenplay - I had no idea what a screenplay was.
"We started writing Dogtooth on weekends. We used to work in coffee shops and restaurants."
Efthymis Filippou with his honorary award Photo: Courtesy of La Mostra de Valencia
Even now, in front of a full house at Valencia's Filmoteca cinema - which is showing a retrospective of his films as part of the festival - he seems slightly surprised to find himself at the centre of attention. He also plays down any notions of either cinephilia or burning ambition, explaining that, where agreeing to work on Dogtooth is concerned, "I did it for myself with the intention of doing something for myself outside the office."
This reticence is also evidenced by the fact that despite Dogtooth being selected by Cannes in 2009 - where it won the Un Certain Regard section and the Youth award - and going on to be shortlisted for the foreign language Oscar (where it lost to out Susanne Bier's In A Better World), Filippou didn't quit his day job until a year later.
"Since then," he says, "I try to make a living out of cinema in Greece - which is not a very easy thing to do."
Easy or not, he sees to be making a square go of it, having been a collaborator on many of the films from the Greek new wave to make it to the festival circuit - not just those with Lanthimos but also Babis Makridis' Pity and Athina Rachel Tsangari's Chevalier - both movies that, like much of his output with Lanthimos, take a particular interest in masculinity and societal construction.
Filippou says he isn't keen on over-analysing his characters before he starts writing - something he shares with Lanthimos, which is why he likes working with him.
"We always start with a fact, with someone or something that's real," he adds, "We choose common human topics and common human stories and we try to exaggerate them."
He adds: "In reality people are more weird than the weird characters in movies. You cannot reach the weirdness of reality, it's beyond our imagination It's something that films try to imitate yet do not succeed."
Since branching out into screenwriting, Filippou has also worked on plays and books, steps he puts down to not coming from a cinephile background.
"I wasn't a film fan," he said. "I watched films but as a common viewer. I wasn't obsessed with film."
Speaking about diversifying, he adds, "I think it would be extremely boring if I only had to do screenwriting for a living."
Efthymis Filippou on Dogtooth: 'When the shooting started, I couldn't believe we wrote something and it had happened'
"We work well together but we do different things," he says. "We have an open relationship.
"The Favourite is a very specific thing that I'm not sure I should be involved with, it's a very British thing. It's normal to want to work with other people."
As for the process of collaboration itself, he says he "feels a freedom when I collaborate with other people". He adds that he prefers for just one of the writing partners to write the script itself, preferably not him, and for the other to offer comments and amendments.
There has been quite a bit of debate since Dogtooth was made about its similarities to 1973 Mexican film Castle Of Purity (El castillo de la pureza), directed by Arturo Ripstein. Filippou insists it's purely coincidental.
"We heard about that film during the editing [of Dogtooth] and it was quite a shock, to tell you the truth," he adds.
"Everything has been done before, but on the other hand, it's really sad when you find out. We decided not to see the film. To tell you the truth, I still haven't see the film."
He admits to having a soft spot for Dogtooth because it was the first film he wrote that got made, but he thinks for a while before telling an audience member which is his favourite, singling out his second collaboration with Lanthimos - Alps, a typically claustrophobic and black tragicomic tale of a group of people who mimic the recently deceased in a bid to help the family grieve.
"Alps was a really hard period for me back then. The shooting was really hard and the process was really hard. I remember that period and it was quite intense so it's a bit more loved than the others."
Although he categorically rules out directing, because it is too hard, he does admit that seeing what his collaborators produce with the scripts he has helped create can come as a shock.
"I'm always surprised," he admits. "It was quite harsh at the beginning - it's always different from what you have in mind."
He adds that as the years go by, he feels as though he should be more prepared for it but he isn't, saying nothing is ever how he has imagined it, "from the face of the protagonist to the shoes of her husband".
Throughout the whole event, his dry sense of humour is evident, not least when a member of the audience says he can't understand why people find his films funny, going on to recall a scene in Alps, in which a man is giving a woman, pretending to be his wife, oral sex, pauses to correct a 'line' she is supposed to recite. Far from being perturbed, Filippou notes that everyone's sense of humour is different before speculating that he is the person who plays the man in the film and perhaps the humour doesn't work because he's simply not a very good actor.
Humour, though, is most certainly important to him.
"In real life, you can't have drama without humour," he says. "Every time I write something and ever time something bad happens it has to be in a way that has humour it in it. I hate being serious. I always like things that make me laugh and cry at the same time."
As for what's next, he's not ready to share details beyond the fact that it is another collaboration with Lanthimos. "We're working on something," he says "but it's at the beginning."