Zak's guilt was revealed as he had a tense showdown with Cain (Jeff Hordley), urging the mechanic to stop telling lies to the police and clear Jai Sharma's name.
Here, Steve Halliwell - who plays Zak - chats about his character's crime and what happens next.
When did you find out that Zak was the culprit?
"It was some time in October when our producer Stuart ran it past me. They still weren't certain it was definitely going to be Zak, but he ran it by the heads of ITV and convinced them. I think it was Stuart's idea that it was going to be Zak - he fought for it because he felt it was most unlikely and would be the best surprise for the audience."
Were you surprised that Zak would go to such lengths to rein Cain in?
"Yes, in a way, but also I was glad because I've been quiet on-screen lately and you can't get more involved than that - almost killing your son. So I was pleased that there was going to be some real drama to do. Actors like acting. Sitting in The Woolpack is alright for a bit, but you want to do some acting and I knew this would definitely demand that."
"The way we've done it and it's scripted - Zak says to Lisa that he just saw red and couldn't stop thinking about the stuff Cain had done, but he hadn't intended to hurt him as badly as that. Zak tries to apologise to Cain, but of course it's futile - he nearly killed him. I think he's quite ashamed of how much he lost the plot."
What drove him to it?
"It's been an ongoing thing. Cain's been humiliating Zak, saying he was past it. He punched him in the face saying, 'You're an old man, you can't do anything about it'. Zak was always a strong physical man and he has got older and is a bit more wary of the younger lads these days, so Cain was rubbing it in.
"Then what he'd done to Amy. Also, Zak was ashamed of the affair with Moira, that Cain was messing up people's families. He's been generally ashamed of the things Cain was doing and no matter what Zak said to him, he just continued being abusive to people around the village.
"In his mind and old-fashioned way of thinking, a father's job is to discipline your son, so he felt it was his duty to do it. And the only way to stop him was to be violent, but he went too far."
"Yes it was difficult, showing concern. Inside my mind, I was playing, 'I can't believe I've done what I've done and I feel sorry for him, but he's still out of order'. In fact, Zak has a line where he says something like, 'Maybe we'd all have been better off if you had died'.
"So I had to play that to some extent, but also family emotions are never black and white. There are grey areas where you love them but you hate them - I think most people can understand that. So it was quite complex, emotional stuff to do and trying not to give it away.
"I was conscious of not giving it away, I kept asking the various directors - what was I showing on my face? Disturbed concern hopefully, but without knowing what Zak's disturbed concern is. I was usually in those scenes with a group of characters so the focus wasn't on me. They avoided close-ups on Zak so that we didn't signal him out with any significance in those aftermath scenes."
Has it been difficult keeping it secret so the outcome wasn't spoilt for fans?
"It has! I'm not good at secrets, so it was difficult. I told immediate family, but then I wished I hadn't because the drama is taken out when you know who it is, to some extent. Especially because the episode when it puts the blame in so many places was very good. My wife likes knowing though, so it won't spoil it for her!"
Do you think Zak expected Cain to turn him in when he gained consciousness in hospital?
"I think he certainly thought there was a good chance he would, because he never does the right thing by anybody - so if he could drop his dad in it, he may well do. If he wasn't going to be able to let Jai take the rap, he'd pay his dad back for almost killing him.
"Zak has always used physicality from time to time. In the past, we've seen him take his belt off to his son many years ago - that's the kind of man he was. But this time he's gone too far. Also, Cain blames his father for the way he is. So yes, I think there was a good chance Cain might laugh at turning his own dad in and find it funny."
"Yes, there are certain principles - even though he lost the plot, he thinks, 'I wouldn't be any sort of man if I let an innocent bloke like Jai go down for attempted murder and a long spell in prison'. He couldn't let it happen.
"Cain points out to him all the things he'll miss, like his daughter growing up. He goads him that he won't turn himself in. But in the end, Zak says 'I don't care, I'll face it all, I've got to do this, if you don't tell the police the truth, I'm going to have to'."
Do you think Zak regrets or could come to regret what he's done?
"Absolutely yes, the fact that he went too far and it didn't seem to change him. The first thing Cain thought about when he woke up in hospital was how to set Jai up, so he's still playing his evil games. It wasn't the answer."
What do you think this will mean for Zak and Cain's relationship in the long-term as father and son?
"Well it's obviously out of my hands, what they write. But I think it would be very, very difficult for them, unless there is a radical personality change in Cain. He'll carry those resentments for the rest of his life I think, that his own father nearly killed him. It's difficult to forgive, certainly for a man like Cain who is a dark force anyway. I've not noticed much forgiveness in him, I don't think."
Is it nice to be at the centre of such a big story?
"Yes, I've heard it's a big year ahead for Zak and it's very nice. I'm on the periphery of other people's stories most of the time which is fine - you can't have stories all the time. But this is nice.
"I think Zak getting testicular cancer was the last major storyline, also Lisa's rape. There are always stories around the Dingle family which is great, but it hasn't been about Zak for a while so I'm really looking forward to it."
> Emmerdale Zak revealed as Cain's attacker
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