It’s a film that is surrounded by as great a veil of secrecy as Tom Cruise’s last film but one - Stanley Kubrick’s Eyes Wide Shut. But to hear Cruise’s producing partner, Paula Wagner, tell it, it’s quite simple. Vanilla Sky is best understood, she says, as a cover version. Which makes a lot of sense, given director Cameron Crowe’s obsession with rock ‘n’ roll. To be more specific, says Wagner, it’s Crowe’s cover version of Alejandro Amenabar’s Spanish film Open Your Eyes.

“We saw Open Your Eyes and we had wanted to work together with Cameron,” Wagner recalls on a somewhat haphazard cell-phone link to a location that, says her assistant Matt, is “kind of hard to reach”. He should know: he’s been trying to do just that for the past hour and a crucial meeting is getting closer and closer as we finally get to talk.

It is some 10 weeks before Vanilla Sky’s US opening (December 14) and things are, as ever, pretty busy at Cruise/Wagner Productions. It was probably much the same a couple of years ago, when the project first began to evolve. Cruise and Crowe had, of course, worked together before: on the latter’s directorial breakthrough, Jerry Maguire. They had remained friends. Then they both saw the Spanish film.

Wagner recalls the process as being pretty swift and decisive after that. “Cameron said ‘I’ll write and direct; Tom will star’ - and it happened!” she claims. “We all loved the film very much and these guys, Tom and Cameron, worked on the script together. Cameron wrote it - masterfully, I might add - and as he says, it’s an adaptation: what Cameron calls a ‘cover’ of a great song.”

Even the normally affable Crowe refused to be drawn as to what the film was about when Entertainment Weekly magazine asked him for a synopsis in its standard ‘Preview’ style of 10 words or less. “A man on a journey… look, I need way more than 10 words,” he joked.

In the meantime, Amenabar brought The Others to Cruise/Wagner, a film which was released in the supposed dog days of August this year and held onto its place in the top five at the US box office for the next eight weeks - the first film this year to do so. If any film starring Nicole Kidman could be called a sleeper, this was it.

A haunted-house thriller, it is the fourth of five films to date from C/W (Wagner is endearingly vague about what the company’s proper name is, saying only “We use both: Cruise/Wagner Productions and C/W Productions”). Founded in 1992, Cruise/Wagner (or C/W) grew out of discussions between its two principals over the years that Wagner was Cruise’s agent at CAA.

“It evolved,” she says, claiming not to be able to recall exactly when they decided to set up a production company. “We had talked about movies over the years and our philosophies of movie-making. We both saw films very much the same way, even though I was an agent and he was an actor.

“Tom had always had a natural feel for the whole of the movie and I did too, so we formed a company. I don’t remember the exact conversation but there were a series of them. We both had a passion about films, about the kinds of movies we wanted to make and how we made them and our beliefs in films. And that kind of joined us towards putting together this company.”

Wagner had been managing Cruise’s career since “the very beginning: right before Risky Business”. But she had already had another career of her own by then. Raised in Ohio, she fell in love with film and “those ladies of the cinema - Hepburn, Bette Davis, Vivien Leigh - and all the great epic films of the forties that you’d watch on television”. I guess Ohio can be like that.

“I wanted to be in movies, but it was this kind of kid’s fantasy,” she laughs, “but it evolved into going to New York and being on stage. There was a point in the seventies where I had actually co-written a play and was working on Jean Cocteau’s Orpheus and doing all kinds of, you know, avant-garde theatre. Then I came out to California and continued to do some acting. But I had other interests and one day I became an agent.”

It was in fact Wagner’s own agent, Susan Smith, who made this suggestion, beginning a process of transition that would lead from Cocteau to Cruise, from Orpheus to Mission: Impossible. Wagner worked with Smith for two-and-a-half years, then moved over to CAA where, in addition to Cruise, she also managed the careers of Demi Moore, Liam Neeson, Oliver Stone and Robert Towne - not, one would guess, the easiest group of people in the world. Wagner was obviously a very good agent.

Since the setting up of Cruise/Wagner, she has revealed herself to be a pretty accomplished producer, too. The company settled in at Paramount, with Wagner moving into an office that had previously been occupied by studio chief Sherry Lansing (and, before that, by Lucille Ball). It was, she has said, a film-makers’ studio, and she and Cruise have no intention of moving on.

Inevitably, there have been suggestions that C/W wouldn’t exist without the ‘C’. And, in a sense, that is probably true: Wagner herself once famously listed the reasons why cinema owners should book the company’s first film, Mission: Impossible, as “Tom Cruise. Tom Cruise. Tom Cruise. Tom Cruise.”