"I wanted to film in late October. It's the most important moment in the year to catch the magic of the city. The sun is low in the sky, and has a silky softness that reflects on the water and makes the colours of the riverside jump out with incredible intensity," he said.
Last year, his principal actor from an experimental theatre group was busy that crucial week. The year before, Nunes was finishing another movie. But when the moment finally came the other day, to his horror and despair, it rained for days.
"When we finally got everything together to start shooting, the wind and rain were so bad that the light was appalling and worst of all the river was so churned up the ferries stopped running."
Lisbon, with its steep narrow streets, is a traffic nightmare, and cherishes its idiosyncratic transport solutions that include trams, about which Nunes has also made a film, and taxis - his next project. But the historic thoroughfare that commands the city is the wide Tagus river, down which 15th-century galleons sailed in their quest for a sea route to India for trade and conquest.
Lisbon's finest monuments face the glinting estuary, offering mariners their last glimpse of home, of Europe, before they set sail for months or years at sea. The magnificent Praca de Comercio has only three sides, the fourth is open to the river. The best views of the city are from the water.
So it was that after nail-biting days of leaden skies and lashing rainstorms, we piled down to the dockside at the first sign of a break in the clouds, to catch a ferry to the southern shore.
Ferries, like all Lisbon's public transport, are overwhelmingly carriers of the working class. Posh people in the stratified society prefer to endure gridlock in cars rather than jostle among shrieking teenagers, housewives laden with fish, or majestically cool Africans from the former colonies.
The crossing to the southern suburb of Cacilhas takes 10 minutes, meaning a hectic burst of activity for the film crew, and a moment of panic when Nunes discovers that the vessel that brings us back is more modern and less scruffy than the one that took us out, ruining continuity. "Well, we'll just have to focus in tight," he said grimly.
As the afternoon progressed, the sun, now almost horizontal, suffused the choppy water with pearly silvery light, giving this stretch of the Tagus the colour and texture that locals call "Sea of Straw". Nunes paused while his cameraman changed a reel. "This is the moment I like best. There's a sudden stillness, it's as though everything stops. You're on a floating lookout, suspended, and your head fills with thoughts of absence, voyages, farewells. It's a disconcerting feeling."
Lisbon's legendary melancholy - its saudade - is attributed to its maritime tradition and its history as an port. "All that waving goodbye and longing to return," laughed Nunes. "But the river also symbolises freedom, when you slip free from the bustle of the city. I want to catch that feeling of marginality, of ambiguity."
Two years ago British engineers built a fine 18km road bridge to span the Tagus near the site of the Expo '98. That helped relieve the intolerable traffic on the old bridge downstream. A year later, a railway was slung beneath the carriageway of the old bridge. But none of these smarter alternatives detracts from the ease and romance of crossing from one side of the capital to the other on a 50p ferry trip.