Sigourney Weaver, 73, is playing a 14-year-old alien in Avatar: The Way of Water

You're only as young as you feel - or in Sigourney Weaver's case, you're only as young as your teenage on-screen character.

While the 73-year-old is returning to the Avatar franchise in the anticipated sequel, rather than her previous turn as the late Dr. Grace Augustine, she's portraying an amphibious alien who's just 14 years of age.

Now, even with today's CGI trickery, it would be tough transforming a 70+ actor into a young teen.

But as we all know, the characters in Avatar are 10-foot-tall, blue-skinned humanoids built from some of the most advanced visual trickery in cinematic history.

And since director James Cameron was so desperate to bring Weaver back into the fold after her previous character was killed off in the first film, he found a way - even if it is out of the ordinary.

Fans will remember Weaver played Sully's superior Dr Grace Augustine in the first film.

Since Augustine died at the hands of Stephen Lang's villainous Colonel Miles Quaritch, some may be surprised to find her name on the cast list for the sequel.

Well, it appears Weaver is actually playing an entirely new character this time around.

20th Century Fox

In Avatar: The Way of Water, she portrays Kiri, Jake Sully (Sam Worthington) and Neytiri's (Zoe Saldaña) adopted daughter.

Speaking about the switch in an interview with You, Weaver said: "I thought, 'Well, Jim [James Cameron] is smart. If he thinks I can do this, let’s give it a whirl!'

"I don’t think John Wayne was asked to play a 14-year-old when he was in his 70s. It’s something new."

Reflecting on her own youth, she continued: "I think it’s a great age, you know? Kids are so imaginative at that age – they’re so sensitive, easily hurt, easily thrilled and they see things so clearly.

"I tried to allow that self-conscious, awkward energy [I had] to flow into the character."

Although some people might find it hard to remember what it was like being that age, for a seasoned actor like Weaver – best known for her role as Ellen Ripley in the Alien franchise – it wasn't too much of a challenge.

Alan D West / Alamy Stock Photo

In a separate interview with Vanity Fair, she explained: "I don’t know that any of us is very far removed from our adolescent moment, because it certainly stands out in bold relief for a lot of people.

"I’m not sure how far I’ve gotten away from my teenager, but Jim said to me, 'You can do this. You’re so immature. This is about how old you are anyway.'"

Avatar: The Way of Water drops in cinemas on 16 December, meaning we don't have long to wait... anymore.

In case your memory needs refreshing, the first Avatar film was released back in 2009, meaning we've been waiting for the sequel for a whopping 13 years.

Cameron, now 68, told Entertainment Weekly that the sequel to the blockbuster has been in the works since 2012, but it wasn't until 2017 that filming actually began.

The film is aptly set around 14 years after the original, and its synopsis reads: "[Former human] Jake Sully lives with his newfound family formed on the planet of Pandora.

"Once a familiar threat returns to finish what was previously started, Jake must work with Neytiri and the army of the Na'vi race to protect their planet."

20th Century Studios

The filmmaker explained that the biggest challenge in making the movie came from the fact that it's set underwater, which, as you can imagine, made its creation a lot harder.

In fact, a specially built 900,000-gallon tank was required for filming - but the producers will be getting their money's worth as it's also being used for further sequels in the franchise too.

As well as Weaver getting into a teenage mindset to portray her character, she also trained herself to hold her breath for six and a half minutes to help film underwater scenes.

Is there anything she can't do?