Richard Lester, the director who brought the Beatles to the big screen, has revealed his determination to capture the Liverpudlian soul of the band in his film A Hard Day's Night, made in the months before the group became internationally famous.

"It had to come from Liverpool," said the 82-year-old this weekend, before the London premiere of a digitally-restored version of his influential film, first released 50 years ago at the height of Beatlemania in Britain.

Lester, who is widely credited with inventing the music video, as well as bringing a surreal brand of British humour to the cinema, told the Observer that he saw the band members change after their pivotal first visit to America and appearance on the Ed Sullivan Show in February 1964. "When they came back from America, I didn't notice that much difference at first," he said. "It was a gradual process, and there were other reasons for it than just international fame, pharmaceutically speaking. But there was just a gradual sense they were developing, musically, as people and in their confidence."

Lester, who went on to make Help! with the Beatles, then the Three Musketeer films and the hit Superman films of the 1980s, said he had wanted A Hard Day's Night to capture the essence of the band's natural wit, so he chose Merseyside screenwriter Alun Owen to write the film. "We had all started working on this fictionalised documentary about them before they went out to have their watershed moment on American television."

John Lennon, the director said, remains one of "two or three people to have shaped my character", adding: "John did not suffer fools gladly, and I probably fell into that category. He always wanted to skewer any pomposity around him, and there can't be any more pompous person on a film set than the director. But I could take all his criticism."

Paul McCartney was more difficult to handle because he was so enthusiastic, recalled Lester. "He tried harder than he should have." Ringo, who has a key solo sequence in the film, proved himself more than capable of projecting his sympathetic persona for Lester's camera, but it was George Harrison who was the easiest to direct. "George was the most effective actor. He attempted less, but he always hit it in the middle, so I knew what I would be getting."

When it came to sticking to the script, the Beatles were professional, Lester said. "They left all of that to us. They would have complained bitterly if they thought it was wrong, I think, or if they felt we didn't have their interests at heart, but I did my best to show on screen what had impressed me so much about them – to recreate their "all for one and one for all" attitude."

Lester said the group functioned like "a four-headed hydra" and did not trust people easily. "But they always had the ability, if they found someone they thought they could trust, to let them get on and do it for them. I was blessed with being one of those people for a while. And George Martin was certainly one, and Brian Epstein too. But with me they soon felt they could control things on their own instead. And when that time comes, you just say: 'Thank you very much, it was a lovely ride.'"

Although Lester was to go on to make the film How I Won the War with Lennon in 1967, he remembered noticing "the change in them all" after Help!. "They had earned the right to do it their way."

The director won the prestigious job with the band because of his television work with the Goons. After meeting Peter Sellers and Spike Milligan when he arrived in London, he had been entrusted to bring their successful radio comedy to ITV. As a young man in his native America, Lester had been a fan of the silent film star Buster Keaton and of the television comedian Ernie Kovacs, so he believes his interest in anarchic comedy was as much due to an American tradition as to a ready understanding of quirky British humour.

Lester's lasting impression of the Beatles, he said, was of their generosity to each other. "They had that great thing of gangs that when one of them was stoned, or hungover, they automatically protected them. The others would do the jokes and then the next night it would work in a different way. I watched it a great deal and there was a sense of "us against the world". As an only child myself, I found it absolutely staggering."