Richard Curtis and Bill Nighy on kissing lessons for the cast, the plots that didn't make the cut – and the crowds of blokes who suddenly turned up to see Billy Mack's girl band

Richard Curtis, writer and director

This film is my Pulp Fiction. I love multiple storylines, but I soon realised how tricky they are. At first, we had 14 different love stories, but the result was too long, so four ended up going, including two we'd actually shot. One was based on a poster in Alan Rickman's office of two women in Africa. The camera actually went into the poster and heard them talking about their daughters' love lives. Another involved Emma Thompson's son getting into trouble at school and the camera following the harsh headmistress home.

We thought we had a good mix of people who were quite famous and those who weren't. Funny how unbalanced it all looks today, now that Martin Freeman is the Hobbit, January Jones is in Mad Men, and Chiwetel Ejiofor is in 12 Years a Slave. I knew from the start I wanted Hugh Grant as the prime minister and Emma Thompson as his sister. And I wrote Martine McCutcheon's part for her, too. I even called the character Martine, though I had to change it before the read-through so she didn't think she'd already got it. Bill Nighy's was the strangest casting. I had two famous guys in mind to play the ageing rocker Billy Mack, and I couldn't decide who to ask. But at the read-through, Bill did it so perfectly he became a definite yes. I've never told anyone else who I wanted. I'll put it in my will.

Although all the strands come together in the airport at the end, it still felt like making 10 separate films. It was a massively difficult edit. The order I originally wrote it in didn't work at all, so we had to reorder it completely. It was a bizarre four-month game of 3D chess.

Once you've made a movie, it becomes a diary – an incredibly expensive momento – of the days you spent filming. I have a very perverse view of it when I watch it now. I think: "Oh, it was hot that day, they weren't in a good mood, I couldn't decide what coat they should wear, I had to give them notes on how to kiss."

I've made so many mistakes in my career. I voted against Hugh Grant being cast in Four Weddings. And here, I had so many arguments about toning down Bill's costumes, but Bill was having none of it because they were so fun to wear. Also, in every film I've done, the bit I like most has had nothing to do with me. That's the awful truth. In Four Weddings, it's Auden's poem. Here, it's Emma crying to Joni Mitchell's Both Sides Now. It's the most deeply striking thing in the movie.

I love the end of Colin and Aurelia's plot, too: him leaving his family Christmas to fly to France and jumping around in frustration at the airport when he misses a cab because he was too nice to get in it, meeting her sister and father, the walk through the town, the translation moment in the restaurant.

The song Christmas Is All Around was just a joke, because Love Is All Around had been at No 1 for 15 weeks and I liked the idea of a compromised character like Billy Mack covering it. I feel that today, 10 years on, Billy would be releasing the Calvin Harris remix to make some extra cash.

Bill Nighy, actor

I know it's disappointing, but there wasn't one person I was trying to emulate in playing Billy Mack. You're supposed to give up reading NME when you're 39 and I gave up at 54. I was obsessed with rock stars, so my character is an amalgam of several.

Unlike Billy, though, I've never said fuck on live TV, which is quite a feat because I use the word obsessively. And I've always felt bad about insulting Blue [writing "We've got little pricks" on their poster], but I understand they took it well. I don't think it did them any harm. And it wasn't true – it was a filthy lie from Richard! So we were safe. I also felt bad about the having-sex-with-Britney-Spears gag. Luckily, or unluckily, I've never met her, so I've never had to account for myself.

Richard is a true believer. He doesn't make romantic comedies in order to, well obviously he wants to make money, but it's real and he finds different ways of expressing it. Love Actually is an old-school ensemble film, all the stories are good value, and it's entered the language, and because it's Christmassy it's perennial.

We didn't all film together, but we had a big trailer park for all the cast. There were so many famous people in there, we used to talk about being on Liam Neeson Way or Emma Thompson Road or Hugh Grant Avenue. And it was a masterpiece of diplomacy, too; we all had the same size and type of trailer.

I recorded the song Christmas is All Around in Abbey Road studios. It was a big deal. I'm not a singer, as you may have spotted, so it was daunting. Billy's band was very cool: the fact the girls were a tribute to Robert Palmer's Addicted to Love video was such a good joke. When we did the filming, the huge hangars at Shepperton studios would fill up: everyone suddenly had some crucial reason to be in this particular hangar. It freaked me out: I had to do my dodgy middle-aged rock jokes in front of all these blokes.

My favourite line – the one they'll write on my tombstone and the one kids still shout at me, is: "Hi kids! Don't buy drugs – become a rock star and they give you them for free."

• Love Actually is on ITV at 10.45pm on Christmas Day.