Karel Reisz, one of this country's greatest directors, died just a couple of weeks before a film dedicated to him is released. Stephen Frears was Reisz's student, champion and friend, so it is fitting that Dirty Pretty Things, Frears's thriller about illegal immigrants scraping a living in the underbelly of the capital, was made in Reisz's honour. As with many of Reisz's films, Dirty Pretty Things concerns itself with those people pushed to the limits of society by race, class, situation or inclination. Reisz's not-so-young apprentice has done his former master proud.

"You can ask me about him, but I might start ... Well, anyway," says Frears, speaking the day after Reisz died aged 76. His usual sprightly manner drops a gear when asked about the Czech emigre who made Saturday Night and Sunday Morning and The French Lieutenant's Woman.

Reisz's last few months had been cursed with serious illness, curtailing the theatre directing he had intermittently returned to throughout his life. For the past few years he had been working on a series of Beckett productions, committing one, a version of Act Without Words starring Sean Foley, to film in 2000. "He kept working, but the past few months have been terribly moving and upsetting," says Frears. "He was a fastidious, discriminating, sophisticated man and he didn't do things without thinking very carefully about them. He was strong and rigorous and I was lucky to have met him."

Reisz and Frears's relationship began in 1965. Fresh from studying law at Cambridge, Frears was swept up by the optimism and talent running through British theatre at the time, and found work at that vanguard of all things leftist and innovative, the Royal Court in London. Like most centres of revolutionary activity in that era, the Royal Court was dominated by committed, politically engaged young men with rampant, combative egos who were out to make a name for themselves, and any new recruits needed a benefactor if they weren't going to be swept under the tidal wave of character that had already swamped the place.

"Good God, it was like the Bolsheviks," says Frears. "You had to have a protector at the Royal Court, you had to have a patron simply to survive. It was so frightening. Bill Gaskill looked after Peter Gill and Lindsay Anderson looked after me. So when Karel came to do a play Lindsay suggested me as his assistant. Then the play collapsed and I was directly moved on to Karel's film, which was Morgan: A Suitable Case for Treatment."

Frears entered this world after its first flush of glory, but at a time when his benefactor was still a director who could do no wrong. Reisz, who had come to film via teaching, was part of a wave of British directors that included Anderson, Tony Richardson, Jack Clayton and John Schlesinger. Influenced by the French nouvelle vague's combination of permissive self-expression and social reflection, the new guard first rose to prominence with Free Cinema - a series of essay-films made at a time when the working classes were seen as anthropological fair game. Much of Free Cinema has dated badly, but Reisz's Momma Don't Allow (1955) - co-directed with Richardson - and We Are the Lambeth Boys (1958) showed an understanding of their subjects sometimes missing in the work of other directors in the scene, who were almost without exception public-school idealists keen to break through the class boundaries held sacred by their parents.

Then the 1960s began with Saturday Night and Sunday Morning. Reisz's film, which stars Albert Finney as a chippy Nottingham factory worker living for booze, fights and sex, came soon after Clayton's gentler Room at the Top, which starred Laurence Harvey as a social climber capitalising on the economic boom and increasing mobility of the late 1950s. In 1959 Room at the Top heralded the advent of a new era, but in 1960 Saturday Night, Sunday Morning threw out the morals of the old one, not least by showing Finney and Shirley Anne Field in bed together.

"Oh, it was a huge hit, gigantic!" says Frears. "It made its money back in the first few days. And it was controversial - banned in Warwickshire! I was too young to know about the impact it had on Karel but I could see the impact it had on the country. Saturday Night changed everything, and it was clearly the work of a highly intelligent man. Room at the Top came from an entirely different place. You have to remember that Karel was identified with the Royal Court and all that went with it. Jack [Clayton] was far more establishment, but Room at the Top is a fantastic film. Who would have thought that to play a northern working-class man you should cast Laurence Harvey, a Lithuanian Jew?"

Saturday Night, Sunday Morning came at a time when Reisz and directors of his ilk were moving away from the political commitment of the late 1950s and into the radical thought of the 1960s. Reisz and Anderson edited modernist film magazine Sequence, went on the Aldermaston marches and plotted their part in the new left. By the time Frears entered the arena in 1965, the mood had shifted. "We were all committed to the overthrow of established values," he says. "And although the political element had waned by the time I signed up, the commitment was always towards seriousness. Lindsay is a more complex figure than Karel, being interested in tradition and innovation, but the characteristic that remained in everyone was high intelligence."

Reisz's Morgan was a case in point; a surreal experiment of a film in which Karl Marx and gorillas were deemed equally important. Frears followed his work on Morgan by assisting Anderson on If ... , which, with its elliptical portrait of public-school rebellion, caught the changing mood as it happened. "We shot the end scene, of Malcolm McDowell killing the headmaster, in April 1968," says Frears. "One of my jobs was to collect images for the collages that the boys have on their dormitory walls, and by May I was cutting out scenes from the papers of what was already happening in Paris. It was ahead of the game."

The influence of Karel Reisz - and of Frears's other mentor, Anderson - can be traced through all of the director's British movies, if not his Hollywood ones. While The Grifters, Dangerous Liaisons and High Fidelity belong to a different world entirely, My Beautiful Laundrette, Prick Up Your Ears and Dirty Pretty Things share an identification with the outsider that can be found in Reisz's Saturday Night, Sunday Morning and Morgan, and in Anderson's If ... and This Sporting Life. With Dirty Pretty Things, the identification is with those invisible people - minicab drivers, hotel cleaners - at the heart of the urban world.

"I now know that when you get into a minicab, the taxi driver has quite possibly arrived in this country by hanging onto the underside of a train," says Frears. "The film is saying that you should take these people seriously, because actually they are far more interesting than you."

We are left with Karel Reisz's work, and the work that it inspired, while Frears is left with the memories and inspiration of a great friend. "I find it very difficult to talk about his films, but I can tell you that he was the most wonderful and wise man ... He learned the hard way. He was very funny, sophisticated, and sly." Frears allows himself a melancholy smile. "The last 40 years of his life were like a love story for me. It's not what you're supposed to say about people, but it's true."

· Dirty Pretty Things is released on December 13.