Fair City star Nigel Mercier has told RTÉ Entertainment that he now knows who killed Karen in the soap, and has challenged viewers to test their sleuthing skills as his character Detective Inspector Yates puts the pieces together in the coming weeks. 

Yates has been a big hit with Fair City fans since the murder hunt began, and now that Seán has been eliminated as a suspect, the detective is turning up the heat on Wayne and Oisín. 

DI Yates interviews Seán

"Yates is coming at it from the point of view that he considers that everybody is guilty unless they prove otherwise with an alibi or circumstances or whatever," said Mercier. 

"So then, obviously, some people are 'more guilty' than others! Once you start lying, you've got to have a very good memory!"

While admitting that his character appears to be "bogged down in the lies and the unbelievable truth" of the Carrigstown murder investigation, Mercier said Yates is an expert at playing the waiting game. 

"People will reveal themselves, all you have to do is wait," the actor said.

"Somebody is going to trip up and all Yates wants to do is to be there at that moment. Even Deegan can't figure out why he isn't moving a little bit faster!"

Oisín has a lot of explaining to do...

Filming five weeks ahead of the episodes that are currently on RTÉ One, Mercier praised the Fair City writing team for doing a "fantastic job" with how the storyline will play out and the big reveal. 

"We have just got the scripts for it; we haven't filmed it yet," Mercier teased.

"If the viewers are any good at picking up hints, they'll start getting it in about two weeks' time... If they're any good kind of detectives, and they watch very closely, they'll figure out who it is..."

"Yates is in no hurry," he laughed. "He'll get them!"

Here's a sneak peek at Thursday's episode: 

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Fair City airs on RTÉ One on Sundays and Tuesdays to Thursdays.

For more on this week's must-see soap moments, click here.

Catch up on Fair City over on the RTÉ Player.