Fifty years after Monty Python and the Holy Grail redefined comedy, stars Michael Palin and Terry Gilliam look back on the freedoms – and limitations – that shaped the film.

An independent British comedy made on a shoestring by a television sketch troupe? It sounds like a film destined to be forgotten within weeks of leaving cinemas – assuming it reaches cinemas in the first place. But Monty Python and the Holy Grail is still revered as one of the greatest ever big-screen comedies, 50 years on from its release in April 1975. Terry Gilliam, who co-directed the film with Terry Jones, thinks he knows why. "Every time I watch it I'm completely bowled over by how incredibly wonderful it is," Gilliam tells the BBC. "It's still so funny, and I just love everything about it."

The Monty Python team first appeared on TV together in a BBC series, Monty Python's Flying Circus, in 1969. Five of the six members – Terry Jones, Graham Chapman, John Cleese, Eric Idle and Michael Palin – had honed their craft in student comedy societies at the Universities of Oxford and Cambridge. The sixth, Gilliam, had moved to the UK from the US, and provided animated segments which linked their surreal sketches. In 1971, some of these sketches were reshot and compiled into a film, And Now for Something Completely Different, but the Pythons had ambitions to make a bona fide feature film – or some of them did, at least.

"It was by no means unanimous that we should do a film after the television series," says Palin. "John, to his credit, was doing Fawlty Towers, Eric was doing Rutland Weekend Television, but the two Terrys wanted to direct a film, and I loved cinema as well, so that was the only way forward – not to make it three Python shows tacked one after the other, but to make it a full cinematic experience. No other television series had, as far as I know, leapt into cinema, but we thought we'd have a go."

Chapman and Jones have both died (or ceased to be, to borrow a line from one of their most beloved sketches), and Cleese and Idle were unavailable to talk to the BBC, but Gilliam and Palin reminisce cheerfully about Holy Grail – Palin quietly affable, Gilliam cackling with enthusiasm.

Palin mentions that he and Jones had made a pre-Python series of historical sketches, The Complete and Utter History of Britain, and Jones would go on to be an acclaimed medieval historian. However, that wasn't why the team chose to build their first proper feature film around King Arthur and the Knights of the Round Table.

"We had to do something that used all six of us," says Palin, "and of course the Round Table was the perfect template for that because we could each play one of the knights. And also because the Holy Grail legend was something that everyone had heard of, but nobody really knew anything about it. You could create any sort of story based around the search for a Grail."

Michael Palin's Five Culture Shifters

The Goon Show (1951 to 1960)

The Goon Shows had a profound effect on me, and I know they did on John Cleese and Terry Jones. For me, they represented a moving-on from my parents' preferences for television and radio. It was just the profusion of ideas – half an hour full of strange images, weird noises, amazing sound.

Heartbreak Hotel by Elvis Presley (1956)

That was a culture shifter for me, because unlike most of the pop songs I was listening to at that time, it made me think that this was something for my generation, and not the generation before.

Lady Chatterley's Lover by DH Lawrence (1928, published in 1960)

These are rather personal choices reflecting on my relationship with my parents. When they got a copy of Lady Chatterley's Lover, it made me feel better towards them. It was a great breakthrough for me to know that they were as interested in sex as I was!

The Rocky Horror Show by Richard O'Brien (1973)

John Cleese invited me along to see the live show in a small theatre in the King's Road. I was quite gobsmacked by this funny and outrageous celebration of the gay world – and there wasn't much of it around at that time. I remember thinking, wow, I've grown up a bit, having seen this.

The London Olympics opening ceremony by Danny Boyle (2012)

It wasn't saying "good old Britain" or "imperial Britain" or anything like that. It was about our way of life in Britain today – and done in such a creative, inventive way. To have a whole sequence about the National Health Service, instead of fireworks and pompous costumes and flags and endless marching........

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