Spanish producer Antonio Saura has unveiled the first feature film out of his start-up outfit Zebra Producciones, an innovative reality show spin-off shot entirely in secrecy and starring the cast of Gran Hermano, the Spanish version of hit TV programme Big Brother.

According to the film's director, Antonio Hernandez, the project - titled El Gran Marciano - marks "the first time in the history of cinema that the protagonists [of a feature film] didn't know they were shooting a film." A crew of 130 people and 36 cameras were employed to document the reaction of a group of friends who encounter a downed spaceship and accompanying extraterrestrial.

Thanks to a series of scripted tricks, such as false news reports and actors contracted to play secondary roles, most of the protagonists - gathered together in a remote hotel on a press junket following their participation in Gran Hermano - never caught on to the hoax. Those who did were immediately incorporated into the production.

The film, kept secret from the press until this week, required an 18 week shoot and a $2.1m (PTS400m) budget.

Marciano has already aroused much interest in Spain, where the cast members from Gran Hermano now form a part of the popular culture. According to producer Saura, plans are underway to use some of the more than 300 hours of footage from the shoot for various interactive spin-offs on internet and TV.

Saura, the former head of the Media Business School, launched Zebra last April with backing from Jose Velasco, CEO of heavyweight TV producer Zeppelin. Also in the works at Zebra are pan-European co-production Semana Santa, starring Mira Sorvino and Olivier Martinez, Jorge Perugorria-starrer Mal Trago from Antonio Galeano, and Miguel Albaladejo's next film, titled Rencor.