The murder-mystery drama gripped viewers on a weekly basis as more than nine million fans waited with baited breath to discover the identity of schoolboy Danny Latimer's killer.
Tennant told The Observer Magazine: "We were taken mildly by surprise when it became a national talking point like that."
The 43-year-old actor also revealed that Broadchurch was initially intended as a one-off and the cast, which also includes BAFTA winner Olivia Colman, were only contracted for the one series.
Broadchurch. Why Broadchurch, again? It was never the intention to do it more than once," he explained.
"It was always written as a one-off. And we actors were all contracted as a one-off: there was no sense it was going to go again."
But creator Chris Chibnall managed to convince Tennant and Colman, as well as Jodie Whittaker and Andrew Buchan, to reprise their roles for a second series - which begins in January.
While plot details about the new episodes have been kept tightly under wraps, the second series is "absolutely propelled by series one", says Tennant.
Tennant and Colman return to Broadchurch as detectives Alec Hardy and Ellie Miller. Buchan and Whittaker, meanwhile, once again play Danny Latimer's grieving parents Beth and Mark. Arthur Darvill also reprises his role as local vicar Paul Coates.
Joining the cast are Dexter actress Charlotte Rampling and comedienne Meera Syal.
