“Gandalf is in Middle-earth to keep an eye on everybody, and that can be a rather serious matter,” says Ian McKellen, not all that seriously, on the phone from his London home the other day.

McKellen, of course, is Gandalf - the wise old wizard and guiding spirit in “The Lord of the Rings” trilogy, and now in “The Hobbit: An Unexpected Journey” prequel trilogy, too.

“The Hobbit: An Unexpected Journey,” Peter Jackson’s hugely anticipated first installment, shot at 48 frames per second (twice a film’s normal speed) and in 3-D, opens Friday.

Set, as J.R.R. Tolkien wrote, in a time “between the Dawn of Færie and the Dominion of Men,” “The Hobbit” follows the furry-footed, reluctant hero Bilbo Baggins as he embarks on a mission to claim a huge treasure guarded by an evil dragon, Smaug. He is joined on his quest by 13 dwarves, and here and there - when crises arise - by the mysterious, mischievous Gandalf. “The Hobbit: The Desolation of Smaug” and “The Hobbit: There and Back Again” are to be released in late 2013 and 2014, respectively.

Although it has been a dozen years since McKellen donned the wizard’s hat for the “Lord of the Rings” trilogy - and although “The Hobbit” takes place more than a half century earlier in Middle-earth chronology - the actor, now 73, believes age is not an issue.

“Here’s the good news for the fans - or the bad news, rather,” he says with a laugh. “I’m exactly the same as I was before. I hope that I don’t look 12 years older. It’s true, Gandalf is actually meant to be 70 years younger - or is it 60? - but actually he’s 7,000 years old, so who’s counting? I think we’ll be OK. … I don’t know in what sense he could be younger. He’s very much the same old soul.”

That said, McKellen is happy to be returning as Gandalf the Grey, not Gandalf the White - the back-from-the-dead incarnation seen in the later, darker stages of “The Lord of the Rings” narrative.

“Gandalf the Grey was always the guy I prefer,” he notes. “Gandalf the White was driven to do a particular job, whereas Gandalf the Grey is a bit more humane. He likes to party, and smokes and drinks and so on.”

Tolkien published “The Hobbit” in 1937. The children’s book is much shorter, and jauntier, than the trilogy that followed, and McKellen says that Jackson’s prequel reflects that difference.

“The whole atmosphere of the book, the tone of ‘The Hobbit,’ is of a kid’s adventure story, told in the first person by Tolkien who is introducing young people to the notion of Middle-earth. A lot of it is very light-hearted.

“It gets considerably darker in ‘Lord of the Rings,’ where, of course, it’s about saving the world. …

“However, ‘The Hobbit’ films are going to lead onto ‘The Lord of the Rings.’ Peter sees it as a six-part film and so there are dark areas, and a lot of that lands on Gandalf’s door, because he’s the guy who is keeping an overall watch on the quest of the dwarves to regain their land and their gold from the dragon - and that will affect the actual stability of Middle-earth as a whole.

“And so there’s a sense of foreboding, and it’s Gandalf who goes and roots that out whilst the dwarfs get on with having fun, really.”