TORONTO -- The ambitious, centuries-spanning film "Cloud Atlas" tackles such heady ideas as reincarnation, the eternity of love and the very nature of humanity.

Sunday's star-packed press conference for "Cloud Atlas," however, was more concerned with jokes about make-up prosthetics, supposed on-set rivalries and apparent lost-in-translation announcements from its Asian actresses.

From the beginning, "Cloud Atlas" star Tom Hanks established the lighthearted tone by leading his three directors, 11 co-stars and the press corps in a rousing rendition of "Happy Birthday" for fellow cast member Hugh Grant, who turned 52 on Sunday.

"Very, very nice, thank you very much," Grant said to a chuckling crowd. "(That's) the first nice thing the press has ever done for me."

From there, an overview of just how writer-directors Tom Tykwer, Andy Wachowski and Lana Wachowski managed to wrestle David Mitchell's acclaimed novel "Cloud Atlas" into a screenplay was frequently intercut with jabs from the jovial Hanks and Grant, who never failed to lighten the mood when things threatened to get too earnest.

"I bitterly regret doing the whole film," Grant deadpanned after listening to co-stars Susan Sarandon and Halle Berry gush over how joyful and challenging their on-set experiences were.

"When they offered these parts, I thought, 'Yes! I might show people I've got more strings to my bow than just one. But a) I was wrong, and b) it's just sitting in makeup with plastic applied to your face for hours and hours."

Set in the past, present and future, each of the main cast-members in "Cloud Atlas" play multiple roles as their characters return in different incarnations over the course of 500 years.

A villain in one era re-emerges as the hero in another, while several actors were challenged to switch genders, race and ages in their various roles.

For Hanks, six wide-ranging parts included that of an unassuming hotel manager in 1936 and a guilt-ravaged goat herder in a post-apocalyptic future; Berry's six roles include a muckraking journalist in 1973 and a German-Jewish woman in 1936; while Jim Sturgess' parts ranged from a 19th century lawyer who hides a stowaway slave to a freedom fighter in 2144 Korea.

Mitchell's book presents the six tales as a series of openings that reach a climactic halfway point and are then resolved one by one.

But Lana Wachowski says it was clear such a structure would not work on the big screen. Instead, each of the tales -- among them a historical drama, a romance, a murder mystery, a comedy and a slick sci-fi adventure -- weave into one another as the film progresses.

"The book feels a little more like an anthology and it's a little bit more acceptable in literature to write a book like that," said Wachowski, who along with brother Andy Wachowski gained fame as the writer-directors of "The Matrix" trilogy.

"But for a movie we thought that would be too hard to start a new story an hour or so, an hour and a half into it, with totally new characters. It would be too hard for audiences."

Nevertheless, she said she doesn't doubt that viewers would be able to make the thematic connections themselves when they see the film assembled as one large story.

It's comforting to know that they already have Mitchell's approval, she adds.

"He said, 'You guys do this amazing thing where you take high-brow ideas and sort of low-brow entertaining narrative motifs and you combine them into a one-brow experience," she said chuckling.

"The mono-brow. We try to make mono-brow movies. We don't like the whole commercial, market-driven thing of splitting movies up into art-house or mainstream."

Later, the notoriously private Lana Wachowski reluctantly addressed a question about her decision to finally come out as transgender in a magazine feature.

"I did feel some responsibility to GLBT (gay, lesbian, bi-sexual and transgender) people and a lot of people have been asking me to be more public," said Wachowski, previously known as Larry.

"But we love anonymity, we love our privacy, we don't really think celebrity does much to improve your life, we think it actually worsens your life. So it was a big decision, it took a long time, it took a lot of years."

It was one of the more sombre moments in a rollicking press conference featuring a whopping 13 cast members, among them British stars James D'Arcy, Jim Broadbent, Ben Wishaw and Asian stars Xun Zhou and Doona Bae.

When Zhou was asked in Mandarin about the difference between working with an A-list Hollywood cast and actors in China, her resulting 40-second answer went over most journalists' heads.

Until Hanks offered a solution: "James D'Arcy will now translate her answer," he said to laughter.

"She had a lovely time, thank you," D'Arcy said to applause.

The Toronto International Film Festival runs until Sept. 16.