Given the endless list of TV channels on offer these days, it takes a lot for a programme to cause a real commotion. But Broadchurch managed that - and more - when it became a national obsession in 2013.

Almost 10 million people tuned in to the final episode when the killer of 11-year-old Danny Latimer was revealed - but the drama hadn't debuted to such a huge fanfare.

"We started life as a little show that nobody had heard of. Then, after a few Monday nights, we started being talked about all over the place. It's been a whirlwind," says the writer and creator Chris Chibnall.

"In the time we've been off air, we've never stopped working on the show's return," says the playwright and TV writer, known for his work on the Doctor Who spin-off Torchwood.

"I knew from the start there was another story to tell in the world of Broadchurch: so much so that I'd pitched it to Peter Fincham, director of television at ITV, in our very first meeting when he green-lit the first series of the show."

And that, he says, is the story viewers will see when the second series begins tonight. All Chibnall will say is that series two is a "different journey".

"It's a new story, and there's a different engine under the bonnet. Some characters return, others don't. And there's a set of new characters for you to get to know," he continues, referring to the likes of Charlotte Rampling, Marianne Jean-Baptiste, Eve Myles, Meera Syal and James D'Arcy.

David Tennant reprises his role as DI Alec Hardy, who arrived in Broadchurch still haunted by the collapse of a previous murder case and, despite the Latimer murder mystery being resolved, hasn't left the Dorset seaside town when series two opens.

"It's a very different type of story," notes the former Doctor Who actor, who recently appeared in the American version of Broadchurch, titled Gracepoint.

"We all found it hard to predict how Chris was going to tell a story faithful to season one. without underselling the veracity of it. It would have been ludicrous and a bit disappointing to discover another body on the beach and begin another eight episodes of whodunnit. He absolutely doesn't do that." Tonally, it's the same show, but structurally, it's completely different."

The father of two says he "loved" being back in Dorset.

"West Bay is the location everyone recognises, and the Jurassic cliffs [where Danny was found] still feature prominently in season two," he says. "This is where Chris lives, so he writes to his locale."

As the investigation into Danny's murder escalated, many townspeople came under suspicion, and it was only after numerous twists and turns that the killer was identified as Joe Miller, the husband of DS Ellie Miller, played by Olivia Colman.

Fortunately for Tennant, he has no problem keeping a secret. "I quite like being the holder of secrets," says the 43-year-old. I'm happy to be in the slightly superior position of knowing what comes next."

His co-star Colman, who resumes her role as DS Ellie Miller, also enjoyed keeping schtum. "I did get a lot of texts and emails from close friends and family, asking me who did it, and I quite enjoyed the power of saying no to them all!" remarks the 40-year-old.

She might have been in on the secret, but the mother-of-two was as keen to tune in as anyone else. "I was gripped! I hadn't seen everybody else's scenes and the cast and crew really kept the secret - it made it special," she reveals.

"We all loved filming and the scripts were brilliant, so you hope people are going to get that, but as the audience built, it was incredibly flattering that people loved it as much as we did."

Despite her huge success (aside from her win for Broadchurch, Colman's received two Baftas for Twenty Twelve and Accused), the actress was nervous about meeting Charlotte Rampling.

"She's a proper legend, but so sweet, and brought everybody a box of chocolates on her last day." As for Myles: "She's told me some of the worst jokes, yet I think about them in the middle of the night and laugh."

A third series has yet to be commissioned, but it'd be surprising if Tennant and Colman didn't sign again.

"Broadchurch is an absolute treat," says Colman. "It was a long job, but "There wasn't one day when I wasn't excited about going to work."

l Broadchurch returns to STV tonight at 9pm.