When Sony Pictures Classics releases "The Imaginarium of Dr. Parnassus" next month, viewers will be treated to a full-blown tour inside the head of helmer Terry Gilliam, courtesy of a mind-blowing blend of miniatures, motion control, bluescreen work and vfx.

What they'll barely glimpse is the thicket of obstacles -- technical and human -- that stood in the way of the film's completion.

The loss of Heath Ledger in January 2008, midway through filming his final role, is only the best known of these. Exactly four months later, while "Imaginarium" was in post, producer William Vince also died, at 44.

Then there was Gilliam's mercurial, exacting personality. "He pulled visual references together and said, 'That's what I want,'" recalls vfx supervisor John Paul Docherty. "But later on, things would change. His mind is always wandering, and there's no such thing as a finished idea for him."

Yet somehow it all came together. Following the loss of Ledger, Gilliam devised a narrative structure that used three new performers (Johnny Depp, Jude Law and Colin Farrell) to play the late actor's character in the scenes taking place inside the film's "Imaginarium" portal, where ordinary experience is transformed into a stunning visual journey. These were filmed on bluescreen stages in Vancouver following the earlier completion of location photography with Ledger in London.

"We restoryboarded some sequences," says art director Dave Warren. "Then we went back and shot the rest of the film."

Many who've seen "Parnassus" assume most of the film is CGI. That's not true, says Docherty. Although the filmmakers resorted to "just about every vfx software known to man" to create certain visuals, much of the work was done with miniatures. Terry was trying for a different style of vfx, more like "Mary Poppins," "Dr. Seuss" and "What Dreams May Come" than "Transformers."

"Currently, vfx is about photoreal landscapes, but that's not Terry," he adds. "If things started looking photoreal, Terry would jump up and down and throw furniture around. It had to look like it was happening in someone's head but remain believable enough that it didn't force you out of the movie."

Achieving this vision required enormous amounts of drawing, painting and pre-vis -- "more than anything else I had worked on," says Docherty.

And it didn't come cheap. Gilliam's ideas were fully thought out but "bore no relationship to any sort of budgetary constraints or the realities of set construction," says Warren. Yet he believes the ambition paid off: "'Parnassus' is a $25 million picture masquerading as a $35 million picture."

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