Broadchurch star Vicky McClure revealed that even the cast have been kept in the dark over who killed Danny Latimer.
ITV1's Dorset-based murder-mystery has proved a major hit on Monday nights, attracting up to nine million viewers each week.
The sixth of eight episodes will be screened tonight, with many scenes shot on location in parts of Somerset including Clevedon, Portishead and Shepton Mallet.
Vicky, who plays national journalist Karen White, promised fans they will be stunned when the killer's identity is revealed.
"Every day I get asked who the killer is – but I'm never going to say," she told the Sunday People.
"We didn't know until the end of the shoot. We filmed it not knowing, which was great because it added to the tension on set.
"When we did find out it was a big surprise. I think people are going to be shocked when it is revealed."
Newsagent Jack Marshall had been the bookies' favourite prior to his suicide in episode five last week. House husband Joe Miller has been installed as the new favourite.
