Keir Dullea and Gary Lockwood in Melbourne on Thursday. Photo: Michael Clayton-Jones

There are many distinctive elements to Stanley Kubrick's 1968 2001: A Space Odyssey - from extraordinary visual effects to challenging themes, from brilliant production design to inventive use of music. One of its most distinctive details is the soft, deceptively calm voice of HAL, the super computer, who suddenly goes rogue during the space mission that takes up the second half of the film.

Yet it's not a voice the people making the movie ever heard on set. Keir Dullea and Gary Lockwood, both 77, played the two astronauts, Dave Bowman and Frank Poole, whose encounters with HAL are part of cinema history. But Kubrick hadn't found the voice he wanted for HAL, they recall, so during production various people delivered HAL's dialogue.

Dullea - whose character hears that famous, ominous line ''I'm afraid I can't do that, Dave'' - often found himself talking to the first assistant director, Derek Cracknell, whose Cockney accent he's quick to imitate. ''It was like working with Michael Caine,'' he says. ''In my case,'' says Lockwood, ''Stanley played HAL with me a couple of times.''

Gary Lockwood in 2001: A Space Odyssey.

At first, Kubrick thought of using a female voice, and calling the computer Athena. Then he was going to cast American actor Martin Balsam, but felt he sounded too New York. Nigel Davenport was on set for a week, Dullea says, before Kubrick decided he was too English.

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It wasn't until post-production that Kubrick settled on Canadian actor Douglas Rain.

Dullea and Lockwood are in Melbourne this week to do a Q&A session after a screening of a 70mm print at the Astor Theatre on Friday, and to appear at Supanova pop culture expo at the Showgrounds at the weekend.

They are happy to talk about the tiniest detail of 2001, to reflect on what Lockwood calls ''a societal game-changer'', and to talk about a director who was open to anything and everything. And there are new things for them to learn about the film after all this time.

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Actors from a Kubrick classic offer details of personal space