In the early 1990s, when screenwriter Richard Curtis and producer Duncan Kenworthy were brought together by Working Title to make a film – a mid-budget comedy to be directed by Mike Newell, something about weddings, strong on morning suits, one-liners, obscenity – Curtis made a suggestion. He told Kenworthy: keep bits. Put aside what mementos you can, in case our film should be worth looking back on.

Four Weddings and a Funeral, now 20 years old, was a hit in America, where it was released in March 1994, and a hit in the UK two months (and $35m in US takings) later. Cast as the lead, Charles, a lovelorn wag who roves his way through the five gatherings that give the film both its title and its neat structure, Hugh Grant was made a star. Curtis and Kenworthy went on to cast him in future moneymakers about poshos in light crisis, 1999's Notting Hill and 2003's Love Actually, though neither, perhaps, had quite the impact of Four Weddings.

Its influence on the British film industry, on romantic-comedy writing, on the pop charts, on funeral readings, on haircuts, was enormous. To mark its emerald anniversary Curtis and Kenworthy, now 57 and 64, have come together with the Observer to look over a fascinating collection of archive material, kept from the days of Four Weddings' production and release.

When he wrote the movie, Curtis was in his late 30s, probably best known for his comedy writing for television, particularly on Blackadder, a co-creation with Ben Elton and Rowan Atkinson that had finished its broadcast run and was finding a second wave of popularity on VHS. "What was perverse," Curtis says, "was that we were left with about eight stills once Blackadder finished. No rehearsal footage. No photographs shot on set. We were uninterested in any kind of archiving at the time. And I'm sure the reason I said to Duncan [that he should keep Four Weddings memorabilia] is that Blackadder was starting to be popular – and we had nothing. Two signed posters somewhere, nothing else."

Richard Curtis (in black) and Duncan Kenworthy look at old scrapbooks and memorabilia of Four Weddings and a Funeral. Photograph: Antonio Olmos for the Observer

Kenworthy took the advice. A long-time executive at Jim Henson Productions he took leave from his job to work on Four Weddings with Curtis. It was his first film as a producer and, justly proud of it, he's kept his archive meticulously, cuttings and keepsakes filling a dozen large folders in his central London flat. A framed poster from the film's release in France, where it was Quatre mariages et un enterrement, hangs on a wall. On the table in front of Curtis and Kenworthy are published versions of the screenplay in Italian (Quattro matrimoni e un funerale) and in Spanish (Cuatro bodas y un funeral).

Kenworthy passes Curtis a script in English, a very early draft, coffee-stained and covered in notes, suggestions, deletions. Inside, a planned sex scene in a limousine is one of the elements messily crossed out. Curtis flicks to the first page – an opening scene you'll probably remember for its massive deployment of the F-word.

"What's sweet is thinking that these words that I thought of as words for so long were made flesh," Curtis says. "The thing is, until the audition process, and until you find the right person, they remain so stubbornly bad words. This reminds me how dead on the page it was, until the right person came in."

They found Grant after seeing, by their estimate, 70 or more possible actors. The actor has been droll, in the years since, as to just how low he was in his career at the time. "I was always the Nazi brother," he once said of his film work to date; when the Four Weddings audition came around, Grant had taken work teaching the French actor Juliette Binoche how to do an English accent. He was being paid cash in hand, "like the plumber". Four Weddings, as he often tells it, was a last throw.

Curtis recalls that the jokes in his script seemed to work, suddenly, when Grant came in to read. "When you audition for my films, nine out of ten times, it doesn't work at all. There may be writers where the writing has a particular quality which means that it sounds pretty good with lots of actors, and you're trying to find the best actor. Whereas I think with mine you're looking for a tiny bit of realistic comic flair. And until you get that optimistic, humanistic performance it's really no good at all."Because of a sudden collapse in funding in the middle of 1992, Four Weddings' developmental stage was unusually drawn out. Kenworthy says, "We auditioned for over a year. Not intentionally, but because we couldn't get the film off the ground. The money wasn't there, the interest wasn't there. But we kept on meeting and talking through the winter of '92, auditioning people…" When Working Title eventually found the funding, Kenworthy believes the emerging Four Weddings was in a stronger place.

"Shooting is expensive but talk is cheap. Just by talking and talking we winnowed out all of the ambiguities, the unfunny lines, the misunderstanding about character that can be so damaging. The film wouldn't have been anywhere near as good if it hadn't been delayed."

After years of scheming, shooting amounted to 36 hurried days in the summer of 1993. The weather was temperamental, Grant later recalling battles with hay fever during boiling outdoor shoots and his cast mate, Kristin Scott Thomas, remembering only rain and the smell of damp tents. Kenworthy has kept call sheets and schedules from the time; also a shooting script with a squiggly line in biro that marks creeping and sometimes harrowingly slow progress through filming. Curtis and Kenworthy laugh, recalling a complicated, multi-character scene that ought to have taken hours but was shot by Newell in a corridor in 20 minutes.

They look over on-set photographs of the cast, including one of Grant in bed with Andie MacDowell, the American actor who'd been cast as his love interest, Carrie. And there are photographs of Scott Thomas, who played Fiona, a member of Charles's wider gang, with Charlotte Coleman, cast as Grant's flatmate, Scarlett. Coleman died, aged 33, in 2001. Curtis says: "In most things you work in, when you reach our age, there are sorrows attached. And to see these gorgeous pictures of Charlotte…" He leaves the thought there.

"A fabulous object, says Curtis, as Kenworthy heaves out a huge, leatherbound folder. It's the size and weight of a couple of Britannicas – a shooting schedule filled with miniature detail about the day-to-day demands on cast and crew. The volume, Kenworthy explains, is a relic of pre-computer film scheduling. "Four Weddings was on the cusp of old-style British film-making," Curtis adds. "It was the last movie I worked on that was cut on [original] film stock, not computer. I've always been grateful that it was shot by Mike in such a lively, gritty way. It's a sketch movie, and he directed it as if it were a drama."

Some of their American backers had doubts, clearly, as to what this movie was. Kenworthy has kept notes that were faxed to him across the Atlantic, expressing unease about Four Weddings' bawdier moments, and how sexual content or bad language might hamper the possibility of its broadcast on American TV. "No blowjobs," the fax reminded Kenworthy, and: "Excessive thrusting and screaming orgasms are not permitted."

There were concerns, too, about Curtis's title. Four Weddings and a Funeral, a studio executive argued, "could turn off men. I can't think of anything men would rather do less than go to four weddings back to back." Alternatives were suggested. Might they call it True Love and Near Misses instead? Or Loitering in Sacred Places? Or Rolling in the Aisles? Or even Skulking Around?

Curtis and Kenworthy cackle when they read through another transatlantic fax, picking over a few of Curtis's riskier gags in pained detail. One line, delivered by Grant – "There's nothing more off-putting in a wedding than a priest with an enormous erection" – had caused particular consternation. The fax reads: "Enormous erection alone is very questionable, but a priest having one is even worse. I doubt it'll fly."

Shooting and editing complete, any transatlantic misunderstandings ironed out, Four Weddings and a Funeral premiered at the Sundance film festival in Utah in February 1994. Its first review, published in Variety the day after the premiere, has been carefully torn out and included in the archive.

Really it needn't be here. The review was so glowing, so important, Kenworthy thinks, to the film's first box office successes, that he can recite its first paragraph by heart. "Truly beguiling romantic comedy," the producer says, "is one of the hardest things to achieve. A winning British team has pulled it off…" Here Curtis interrupts. "Well, as so often in life, the experience on the inside was different to the outside."

Hugh Grant and Elizabeth Hurley arrive at the FWAAF premiere party in Leicester Square, June 1994. Photograph: Richard Young/Rex Features

He continues: "The premiere was in Salt Lake City. And my major memory is that, because of the swearing in the first two minutes, all the town council, quite serious Mormons, stood up and walked out. I was sitting two-thirds back with Hugh. It was the first time he'd seen the finished film. And I remember him saying, 'Perhaps it's not going to be as popular as we'd hoped...', you know, as 30 people trooped out past us."

It was just about the last hiccup. Four Weddings was on the march to ubiquity. Not long after the Salt Lake City premiere, Polygram, backing the film in America, was in a position to be able to buy up pages of newspaper adverts, boasting about massive early box-office figures. In one ad (which has, Curtis points out, a rather unfortunate image in which MacDowell is supposed to be doing up Grant's tie, but instead looks as if she's strangling him) it was announced that the film had earned $27,697 per screen during its first five days of release.

That would have been about £19,000 at the time. Grant's fee for the whole movie was £40,000… Curtis remembers having to cut $1.2m from a $4m budget, "just before we started. We had to double up on vicars. And yet once the movie looked as though it might succeed, suddenly, out of nowhere, there was $11m to spend on promotion". Kenworthy: "The only way you can prove that you were right and they were wrong [about budgets] is to make a lousy movie. And when you make a good movie they say: 'You see? You didn't need the extra money.'"

Both Kenworthy and Curtis are cautious when discussing the film's final commercial success (£144m worldwide, in the end). They aren't natural gloaters, and Curtis points out that when they first had discussions about Four Weddings' global prospects, and projections were made as to how much money it was likely to make in each territory, the figure put beside north America was precisely zero. "Our film was small. English. We expected very little when we started."

Four Weddings' British premiere took place in Leicester Square. Curtis can hardly bear to read the cheesy filler he wrote for the invitation, something about the film being "totally and utterly wedding-y". "Jesus," he mutters, moving on to a page of cuttings pulled from the tabloids. Curtis and Kenworthy are surprised and amused to see that the Sun's gossip columns about the premiere were penned by Andy Coulson; in one spread the journalist has been photographed in a frock and wide brimmed hat, acknowledgement of the fact that some guests on the night wore their wedding outfits.

Kenworthy: "That was Richard's idea. It created such emotion. Can you imagine, Odeon Leicester Square, 2,000 people and a good 200 in their wedding dresses? There was such a thrill in the air." Curtis: "God you're a romantic. As if. It was lots of people having to let their dresses out, thinking, 'Fuck I've got fat.'"

The night belonged to one particular dress. "Sexy Elizabeth Hurley couldn't have looked less like a blushing bride," read one of Coulson's columns for the Sun. "The stunning 29-year-old girlfriend of Four Weddings star Hugh Grant drew gasps and wolf whistles when the crowd caught sight of her breathtaking £3,000 black dress."

Curtis and Kenworthy are happy to acknowledge the flukey breaks that helped Four Weddings become as big as it did. Fifteen weeks in which the film's main track, Love Is All Around by Wet Wet Wet, topped the pop charts didn't hurt, for instance; and they class the free publicity created by Hurley's premiere-night dress in this bracket. Good luck. "Mike Newell says he was elbowed to the ground by a stampede of paparazzi at the premiere," says Kenworth y. "Stories around the dress were spun out for ever after."

Outside Britain, the press focused on a different subject. Curtis and Kenworthy come across a glossy American magazine cover, its cover image of a pouting Hugh Grant overlaid with the word: Swoon!

"Now this has taken me by surprise," Curtis says. "The memory of the fuss about Hugh. We're sort of used to him now. We've grown up with him. But look how young and handsome he was. He looks like the lead singer of a band." Kenworthy: "It was the most amazing luck that when Hugh went on the publicity trail he turned out to be incredibly funny." Curtis: "And very like the character of Charles. That doesn't ever happen… I remember him winning his Golden Globe [in early 1995, for best actor in a comedy, musical or animation] and making a speech that was just full of jokes. He was very taken aback, afterwards, when people assumed that I had written it. But in real life he's much funnier than I am."

Something else that Curtis didn't write: the lines by WH Auden, beginning "Stop all the clocks", which are read out by John Hannah's character in the film's funeral scene. "That's what's funny about my films in general. On the whole, the very best bits are the bits that didn't have anything to do with me. Auden is the key emotional moment in Four Weddings. In Notting Hill it's the walk through the changing seasons, which I don't think was in the original script. In Love Actually it's Emma Thompson crying to the Joni Mitchell song. When you write a lot – and there's lots of dialogue in my movies – the bits where the dialogue stops, and something else happens, can often be the most striking part."

The script for Four Weddings earned Curtis an Oscar nomination, when the awards were staged the following year. Grant was overlooked in the acting category, something that obviously still annoys Curtis and Kenworthy; Four Weddings, however, was included among the contenders for best film. Matched against those heavyweights of '94 – Pulp Fiction, The Shawshank Redemption, Quiz Show and Forrest Gump – it was nobody's favourite to win, certainly not Curtis and Kenworthy's. "But we had hope for a nano-second," says Curtis. "When they read out the winner, they said, 'Fff–'"

Kenworthy: "They said, 'Forrr–'"

Curtis: "It was Forrest Gump. But for just a moment…"