Even with less than a month until Star Wars The Force Awakens opens, fans have been speculating as to Hamill's role; The Force Awakens trailers have featured a voiceover from Skywalker and a shot of a cloaked figure who could be the Jedi Knight, but the character hasn't been spotted or referenced anywhere else in any other Star Wars promos.

Hamill confirmed to fan magazine Famous Monsters of Filmland that his iconic character will indeed appear in the new film, saying only that the reboot is "about setting up the next generation of heroes and villains, but the fact that half a dozen cast members from the originals can be there to place it in some sort of context, to be there for the historical side of things, I think it's wonderful".

But why all the mystery?

In a video interview with PC Gamer last week, the actor hinted that a profitable agreement might be the reason for his silence, although his reference was fairly oblique.

"I have something coming out where there's an amount of money that if it leaks because of me, I don't get that payment," he said. "If I keep it a secret until the day it comes out, I get that payment."

Hamill has a reputation for keeping mum, it seems - back in the day he managed to keep the Star Wars franchise's biggest shock a secret, even from his own wife.

"I'm good about keeping secrets. I'm proud of the fact that I knew a year and a half before anybody else that he (Darth Vader) was actually dad-Vader," he told PC Gamer.

"I didn't even tell Harrison (Ford), or my wife, because I thought it would be terrible if she said 'you know what' to a fan and it got out, because (Empire Strikes Back director) Irvin Kershner said 'I know something, I'm going to tell you'.

"'George (Lucas) knows, I know and now I'm gonna tell you, and if it leaks, we'll know it's you', because the line originally was, 'You don't know the truth, Obi Wan killed your father', and he said 'We're going to take that line out and put in 'I am your father'."

Online speculation continues about Hamill's role - is he still alive in The Force Awakens? Is he still a Jedi or has he gone to the dark side?

Hamill, true to his word, only offered still more cryptic clues.

The new film, he said, will be "every bit as exciting for the audience, [but] it's not going to be like it was then, when we're rattling around in the Death Star, and exchanging quips, and jockeying for the affections of the princess. It's going to be age appropriate. The story has moved on, and our purpose in the story is different than it was then."

Star Wars: The Force Awakens opens in Australia on December 17.