Blockbuster reality TV show Big Brother is back with a flashier intro, secret rooms, stiffer challenges and a wild cast

Think again, because Big Brother promises "There's more than meets" the eye this season.

Executive producer Amelia Fisk says that premise applies to the house and the cast.

"You might think you know someone, but there is more to them as the housemates develop," she says.

Her and a "big team" sifted through some 20,000 applications to find just the right blend of people to enter the Big Brother house in 2021.

"It's one of the highest volumes of applicants because it's such a super brand, but also you don't have be good at anything to take part.

"The cast is very diverse. There is someone for everyone to cheer on. They have to draw some emotion from the viewer. In this series, there is character development - heroes become villains and villains become heroes," Fisk says.

Obviously the housemates have to have personality, but Fisk says they also have to have something worth leaving behind.

"It can't just be a holiday, then you are not going to push yourself. We steer away from someone just wanting [social media] followers. We look at occupations too. It might give them the tools to take on the challenges. It's like a football team. You don't want to have a team full of forwards. We try to reflect a cross-section of society in Australia. And we have to get it right."

Big Brother is everywhere, and has more of a personality this year. The housemates hear his voice, but it's "very fly on the wall", Fisk says.

She's not kidding - with 56 rigged cameras throughout the house and 10 camera operators, very little is missed.

So, who will win? As the show is prerecorded, but with a live grand final, the housemates will reconvene to let "you decide", says Big Brother.

BIG BROTHER: Starts Monday, April 26, Network Seven, 7.30pm