Richard Curtis has said that the lack of diversity in his 2003 festive romcom Love Actually now makes him feel “a bit stupid”.

Speaking to Diane Sawyer for an ABC special ahead of the film’s 20th anniversary, Curtis was asked if any parts of the ensemble comedy made him wince.

“There are things that you would change,” said Curtis, “but thank God society is changing. My film is bound in some moments to feel out of date. The lack of diversity makes me feel uncomfortable and a bit stupid.”

Curtis added: “There is such extraordinary love that goes on every minute in so many ways, all the way around the world, and makes me wish my film was better.”

No liaisons are shown in the film which aren’t hetereosexual, although a strand featuring Anne Reid and Frances de la Tour as lovers was shot then edited out.

The film features only one main cast member – Chiwetel Ejiofor – who is not white. In 2017, Working Title, the company behind the film as well as many of Curtis’s previous triumphs, opened a new sixth form school in London to try to improve diversity in the British film industry.

Curtis was speaking as part of an hour-long special, The Laughter & Secrets of Love Actually: 20 Years Later, broadcast in the US on Wednesday. Also speaking to Sawyer were cast members including Hugh Grant, Emma Thompson and Bill Nighy.

The film, in which interconnected tales of romance, friendship and familial love climax around the festive season, met with a mixed reception from critics on its release but was warmly embraced at the global box office, taking $247m.

It was nominated for three Baftas and two Golden Globes and has over time gained a reputation as a seasonal Marmite classic, with some devotees religiously viewing it each holiday season, while other viewers swear off it entirely.

The Guardian’s Peter Bradshaw gave the film two stars out of five on its release, praising some performances, but criticising Hugh Grant’s “icky, disingenuous” voiceover which bookends the film, showing loved ones reuniting at Heathrow airport.

Grant plays the newly elected prime minister in the film, in which he falls for tea lady Martine McCutcheon and stands up to a lecherous US president played by Billy Bob Thornton.

Speaking to Sawyer, Grant, 62, said he initially found Curtis’s script “a bit psychotic”, calling it “Richard on steroids”.

“But the thing is with him, what you have to remember is, when he writes about love, he means it,” Grant added. “And that is quite rare.”

Grant also called the sequence in which his character dances solo through Downing Street “the most excruciating scene ever committed to celluloid”.

Grant has frequently spoken of his discomfort shooting the scene, in which he wiggles through corridors to the Pointer Sisters’ 1983 hit Jump, before being caught by a personal secretary.

“I think I saw it in the script and thought ‘I’ll hate doing that’. No Englishman can dance when they’re sober at 8am in the morning,” he said.

Curtis said he suspected Grant “was hoping I would get ill or something and they’d say, ‘Oh, what a shame to lose that dancing sequence’. He was grumpy but he knew he was under a contractual obligation.”

Grant said he had drunkenly rewatched parts of this film with his wife, Anna Eberstein, and she had commented: “‘Oh look, it’s all about pain, it’s all about suffering.”

Both Nighy and Thompson were particularly acclaimed for their roles in the film, as a washed-up rocker, and a woman who discovers her husband (played by the late Alan Rickman) has been unfaithful. Nighy took a Bafta for his performance; both stars also won Evening Standard and London film critics’ awards.

Nighy, who is in line for his first Oscar nomination next year for his role as a terminally ill civil servant in Living, said the film had been “wonderful to be a part of”.

“It’s amazing the way it’s entered the language,” he said. “I have people coming up to me saying it got me through my chemotherapy, or it got me through my divorce, or I watch it whenever I’m alone.”

Thompson has just opened as Miss Trunchbull in the big screen transfer of the Matilda musical and is also a likely contender for an Oscar next year for her role as a bereaved woman who hires a sex worker in Good Luck to You, Leo Grande.

Speaking on the ABC show, Thompson said Curtis’s “golden heart” was the key driver behind the longevity of the film’s success, as well as his own career. “He’s truly a good person, and in our business that’s something that is to be treasured.”

Grant added: “He is funny, that’s a black and white thing … and [the story] comes from the heart, it’s true.”

Curtis later said: “I do think that the way to think about life is that every day has the potential just to be gorgeous. I think when you get it right, films can act as a reminder of how lovely things can be and how there are all sorts of things which we might pass by, which are in fact the best moments to our lives.”

Other stars of the film who took part in the show included Laura Linney and Thomas Brodie-Sangster. Colin Firth, Liam Neeson and Keira Knightley did not take part.

A short sequel to the film was released in 2017 for Red Nose Day.