It is not just a brand-new Middle Earth that Peter Jackson is creating with his latest fantasy trilogy, based increasingly loosely on JRR Tolkien's The Hobbit. It is also a shiny new promotional process that sidesteps lingering concerns over the bloated nature of the new project – there apparently remain a few dozen weirdos out there still wondering why a breezy, 350-page children's fantasy fable has been transformed into a triptych of three-hour big-screen epics – in favour of a full-scale global fan love-in.

So here we are in London's Leicester Square, linked up by the power of satellite technology to Jackson in Wellington, New Zealand, as well as to other cinemas in New York and Los Angeles, each of which has its own host, an audience of baying geeks in fancy dress and various cast members from The Desolation of Smaug. In London we get Andy Serkis (Gollum), Lee Pace (haughty wood-elf king Thranduil) and Luke Evans (Bard the Bowman). In LA they have Lost's Evangeline Lily, who plays wood-elf warrior Tauriel. In New York, there's Britain's Orlando Bloom, looking quite surprised to have been invited back as Legolas (whom he refers to throughout as "Leggy") from The Lord of the Rings. Jackson oversees proceedings from New Zealand, relaxed and seated with a mug of coffee in paw, hobbity bare feet on display and an unmistakable air of avuncular bonhomie.

"The fundamental thing people should understand is that the voice of the novelist can guide you through the story [in the book]," he says. "In The Hobbit, Tolkien is taking us on this journey. As a film-maker, you don't want to insert the voice of the film-maker … you convey the story through the characters on the screen."

Without all these new and souped-up elves and men of Laketown, the film-maker seems to be saying, the new trilogy would only have been as good as its source material. And that wouldn't do at all. I mean, who even likes The Hobbit anyway?

Lily is more openly apologetic about the existence of Tauriel, unmentioned in Tolkien's tome but apparently an orphan whose parents were killed by orcs. "Once you understand that, you can understand how and why this girl became the lethal killing machine that she is … how she got taken under Thranduil's wings and why she's so passionate about fighting the evil that exists in Middle Earth," she says. "And I'm sorry, but if an orphan story doesn't win you over, then you've got a heart of steel."

Thankfully, soon it is on to the 20 minutes of new footage, and we are not to be disappointed. There's a scene in which Thranduil and Legolas interrogate an orc that is clearly intended to prefigure the events of The Lord of The Rings, but the rest is mostly genuine Tolkien meat and bones drawn straight from the book itself: Martin Freeman's Bilbo Baggins fighting off 'orrible giant Mirkwood spiders who have taken his dwarven companions by surprise; Bilbo again in the chambers of the wood elves, freeing the dwarves so they can travel down the river to Laketown and the Lonely Mountain.

Another scene hints at the "Aragornisation" of Bard, who only has one or two lines of dialogue in the book, but it's not nearly as irritating as some of An Unexpected Journey's pointless additions. Finally, there's a glimpse of Bilbo's journey into the Lonely Mountain itself, and the beginning of his encounter with the great wyrm Smaug. Jackson has played fast and loose with the narrative again here, with Bilbo far more clumsy and incautious than I remember him in the book. Yet the golden hoard of dwarf treasure and the gargantuan, slippery snake-like body of the dragon itself are drawn straight from the most magnificent childhood memories. Seeing them on the big screen in crystal-clear quality is a spectacular moment, and I can't wait to watch the entire film in 48 frames per second.

Even so, I can't help thinking that The Desolation of Smaug takes us rather a long way into Tolkien's story. At this rate, the entirety of the third movie will be the Battle of Five Armies and its (limited) aftermath. In the book, Bilbo pretty much slept through the whole thing, so Jackson will finally have free rein to fill his boots without fear of criticism from Tolkien purists. Perhaps The Hobbit will now end with an invasion of hobbit warriors, or the arrival of Aragorn and his rangers. You wouldn't put it past them.

The Hobbit: The Desolation of Smaug opens on 13 December across the globe, the day after debuting in New Zealand. You'll moan a lot about Jackson's relentless Tolkien tinkering, but, like me, you know you're still going to see it.

• This article was amended on 5 November 2013. The original said Bilbo was travelling to the Misty Mountain, not the Lonely. It has been corrected.