The Imaginarium of Doctor Parnassus, Terry Gilliam’s latest mind-warping fantasy romp, has been going by another, far simpler and more tragic title since Jan. 22, 2008. That’s the day it became known as Heath Ledger’s last film.

About half way through principal photography, Ledger died of an accidental overdose, halting production and leaving a wake of grief for the filmmakers to combat. Gilliam, however, was adamant that the film be “salvaged” in his memory and it is now dedicated to Ledger.

The Imaginarium is a re-telling of the classic Faust tale. Set in present-day London, it tells the story of a deal struck between immortal, thousand-year-old Doctor Parnassus (Christopher Plummer) and the Devil (Tom Waits, in an uncanny and delightful turn). In their battle for supremacy, Parnassus leads a traveling theater troupe that offers audience members a chance to go through a mirror and into the Imaginarium, a magical world where their souls have to choose between good and evil.

Thanks to the Lewis Carroll elements of the story, Ledger’s role was reincarnated by Johnny Depp, Jude Law and Colin Farrell, who each portray different permeations of the character as he travels into and through this dream realm.

Gilliam, in a muted Hawaiian shirt and linen pants, his gray hair sticking up in Bart Simpson-esque spikes while a long, thin tail trails halfway down his back, is a man of explosive energy and vivacious spirit. But even his unwavering enthusiasm dims when he discusses Ledger. While Gilliam still tends to punctuate his sentences with throaty, bawdy laughs, the mention of the young actor brings a calm angst.

The director says his greatest regret is “never being able to see what Heath was going to do behind the mirror. I know he had a lot of tricks up his sleeve. He was building that character so it was a chameleon. Even his accent; it shifts from Australian to Cockney to proper English. He was just this fluid creature who, on the other side, could be whatever. We’ll never get to see that. That’s the thing I miss most about the film, forget about what I miss most about Heath.”

Prodded further, Gilliam unfurls a tender recounting of a friend and collaborator he admired tremendously.

“He was an extraordinary human being,” Gilliam continues. “As an actor [and] as a human being, he had this ancient wisdom. All of us who were close to him never understood how he was so old and wise for a guy in his 20s.”

Gilliam believes that the flashes of greatness audiences witnessed in Brokeback Mountain, The Dark Knight and, now, The Imaginarium, were just the beginning of what could have been an extraordinary career.

“He was becoming more playful, more confident, taking more chances. He was growing. I hate the loss of greatness and potential and that’s what happened with Heath’s death. He would have been unbelievably phenomenal because he was fearless. To be fearless and playful at the same time is quite wonderful. I’d never worked with anyone like that at all.”

Moving forward with the film after Ledger’s death, Gilliam needed to create a cohesive story that also honored Ledger’s memory. He explains that none of the film’s script was changed to accommodate the solution they struck upon; casting actors to play Ledger’s character, Tony, on the three occasions he enters the transformative mirror.

“I cast people close to Heath so they knew him. We gave them DVDs of what we’d been able to assemble of what Heath had done. They arrived, there was no time to rehearse and they did it. It’s as simple as that. Johnny we had one day and three and a half hours to do what he did, which I still don’t know how we did. Johnny becomes this lothario, Jude is the more straightforward, ambitious guy and Colin comes out as a darker bad guy.”

It could be said that each of Gilliam’s movies offers a peek though the mirror into his own imaginarium and the many sides of his personality. But if one were to actually walk inside, what would the visitor find?

“You might not like it,” Gilliam playfully growls, descending into a gale of giggly, hoarse cackles before straightening up and replying earnestly. “I’m basically very reactive, so whatever is bugging me in the world, I try to deal with it in the films I’m doing. I suppose each film is an attempt to invent a world and see if I’ve got it right and then I finish the film and realize, nope, got it wrong. Let’s do another one and try to invent a world that makes sense to me.”

Asked how this film is autobiographical, Gilliam’s mouth curls into a wide grin and his eyes twinkle. “Well, it’s some old fart who’s trying to inspire and enlighten people and no one’s paying attention. How about that?”

The Imaginarium of Doctor Parnassus releases in select theaters Dec. 25.