In the end, it was a less noble reason that inspired McKellen to resurrect his role as Gandalf for The Hobbit trilogy - pride.
"Selfishly, it's because I couldn't imagine someone else playing him," he admits, chuckling. "That was the clincher, eventually, because they made it very clear to me that whatever I decided the film was going ahead - I wasn't a film-breaker. They also made it clear there were plenty of others who would happily play Gandalf. Knowing that, I thought; 'they're not going to get the chance' because I'd invested too much in the role for someone else to come along and take it over, as with Dumbledore in the Harry Potter films."
Any niggling doubts McKellen had about returning were swiftly forgotten when filming began and he realised The Hobbit was not going to be just another instalment of Lord of the Rings.
"It quickly became apparent that this Gandalf wasn't quite the same character as before, in that his politicking is more to the fore - you see him trying to persuade people and manipulate the hierarchy of Middle-earth, in a way you don't in Lord of the Rings - and of course he's a little younger and a bit more sprightly," he reveals. "Not that he'll suddenly be doing cartwheels, because when you're several thousand years old, being 60 years younger doesn't make that much difference."
For McKellen, one of the major attractions, this time, was getting to act opposite Cate Blanchett as Galadriel.
"We hadn't before, but we had two weeks filming together for this and got on terribly well because she's a theatre person too, so we had a lot in common.
"I had two wonderful scenes with her. One was a very personal, delicate scene at Rivendell, in which she expresses her concern for me. There was a wonderful sunset behind me, and she saw a strand of hair out of place and gently reached up and put it back. I touched her hand, and then looked down at her but she'd vanished," he says, wistfully. "Peter recorded all of this and was very taken with it - I thought it might be a bit too sentimental - but it's still there in the movie, I think. So that was a special moment, which reminded me why I came back.
"Also, the emphasis of these films is very different because it's about a hobbit and some troublesome dwarves - who weren't in Lord of the Rings - going on an adventure, rather than trying to save Middle-earth," adds McKellen.
It means that The Hobbit: An Unexpected Journey is more light-hearted than Lord of the Rings. It's also more comedic, with merry moments of tongue-in-cheek humour.
"Yes, there's a lot more comedy because some of the world's greatest comedians are in it, like Billy Connolly, Barry Humphries and Steven Fry," he says. "Peter would occasionally say to us, rather unhelpfully, 'be more funny' when we were filming.
"A lot of the dwarves are extremely funny too but I think the biggest surprise will be Martin Freeman. I think his performance is going to be the major coup because although he's fairly well-known already, as a comic actor, it's going to be a surprise to suddenly see him arrive as a fully-fledged, great film actor, which is what will happen. Once again, Peter Jackson has come up trumps, and got the casting spot on ..."
If he says so himself.
-TimeOut
