The Big Bang Theory is getting a spin-off based on 12-year-old Sheldon Cooper

Big Bang Theory fans sat upright last year when it was revealed the unsociable but quirky scientist Dr Sheldon Cooper would have his own origin story.

Now, actor Jim Parsons has shed more light on the TV spinoff, saying it will be "very different" from the show that made his character a household name.

Parsons told Entertainment Tonight there will be plenty of material for screenwriters to work with, especially given Big Bang's creators have done "such a good job" with his character's history and background.

Indeed, Cooper's rapid-fire jokes in the show often revolve around his childhood - with numerous references to his Evangelical mother and formative years as a child prodigy growing up in Texas.

"It just seems like a really wasted opportunity if you don't decide to explore the origin story with that," he said.

"I mean, they've layered so many things in there over the past decade that is already there to be drawn from. I'm really excited about it."

And if you thought Sheldon's origin story would be a cookie cut-out of Big Bang Theory but set 20 years in the past, think again.

"I think it will be very different than Big Bang, but in a good way," Cooper said, hinting the spinoff could be aimed at a younger audience.

As for any advice the veteran actor has for his younger replacement, Parsons said whoever steps into the role shouldn't be afraid to give teen Sheldon their own twist.

"I would advise that actor to watch me as little as possible and take that thing by the horns and make it [their] own," he said.

Sheldon's origin story is still in the early stages of development, with the lead role yet to be cast.

Big Bang Theory is TV's highest-rating comedy. It's now into its tenth season, with rumours swirling as to whether or not the show will be renewed.

Parsons, along with his co-stars Johnny Galecki and Kaley Cuoco, reportedly make around $US1 million ($1.3 million) per episode.