Related Video: "The Hobbit: An Unexpected Journey" trailer
The actor, who has played Gandalf since Jackson's first "Lord of the Rings" film, became upset after he had to perform the scenes with only pictures of his other cast-mates.
"In order to shoot the dwarves and a large Gandalf, we couldn't be in the same set. All I had for company was 13 photographs of the dwarves on top of stands with little lights - whoever's talking flashes up," he told Contact Music.
"Pretending you're with 13 other people when you're on your own, it stretches your technical ability to the absolute limits.
"I cried, actually. I cried. Then I said out loud, 'This is not why I became an actor'. Unfortunately the microphone was on and the whole studio heard."McKellen is currently on the publicity tour for "The Hobbit: An Unexpected Journey," the first in three films to be made from Tolkien's book.
"The Hobbit: An Unexpected Journey" hits local cinemas on December 26.
Ben Arnold writes for Yahoo! Movies UK & IE