Wellington, July 13 NZPA - New Zealand writer-director Andrew Niccol plans to cast only actors in their mid-20s for his new film, sci-fi action thriller, I'm.mortal.

The film revolves around a society in which ageing stops at 25, Variety magazine reported on its website. Shooting is expected to start later this year.

It will star actress Amanda Seyfried as a socialite in a society where people aged 25 must buy and bank time and the richest can live forever while the poor just die when their money runs out.

The lead male has not yet been cast, but will play a poor man who murders a rich one for his time, and then goes on the run, taking as a hostage Seyfried's character.

"It's a strange casting process," Niccol told Daily Variety. "Even for a small role like a priest or a pawnbroker, where you would normally cast a 75-year-old character actor, I have to find a 75-year-old character actor in a 25-year-old body."

Hollywood Reporter said Niccol's script was the latest entry into the arena of "social sci-fi", a genre that uses one big idea to reflect on societal issues, such as last year's District 9, and earlier works by Niccol, including Gattaca, The Truman Show and S1mOne.

Meanwhile, the Vancouver Sun reported computerised graphics for the seventh Planet of the Apes film, 20th Century Fox's Rise of the Apes, will be done in Wellington.

Production has already started at Vancouver's Mammoth Studios, and Weta Digital will be doing the digital effects for the film, including digitally-animated apes.

Weta -- owned by Richard Taylor and producer Sir Peter Jackson -- won Oscars for its work on Avatar, Lord of the Rings and King Kong. It also did digital effects on District 9, the sci-fi thriller directed by Neill Blomkamp.

Andy Serkis -- best known for voicing and providing the motion capture movements for Gollum in The Lord of the Rings trilogy, and providing movements for the ape in King Kong -- has joined the cast as the head ape, Caesar.

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