Zak has been behaving strangely for some time, and fans see the situation escalate next week as he goes wandering on the moors. Confused and delirious, he then ends up in danger as he stands on a ledge above a terrifying drop downwards.
Halliwell told TV Times that he is enjoying pushing himself with the dramatic plot, which was first pitched to him by Emmerdale's series producer Stuart Blackburn.
He explained: "When Stuart suggested this storyline, I could see the value of it dramatically. Zak is a conventionally macho man who used to be able to tackle any problem, but people like him can go to pieces too. Look at what happened to Frank Bruno.
"It's been a challenge for me to play a different side of Zak, but I must say I was worried about the scenes on the moor. I told Stuart I didn't like heights and had to check out the location beforehand - because I knew that if I couldn't go out onto the ledge, I'd ruin everything."
The actor revealed that he calmed his nerves by meditating on the filming day.
"I did a course in Buddhist mediation some years back," he explained. "I do it when I feel the need and when I want to unwind. I've known people like Zak - men who would never talk about things and shoulder their illness alone, but that's not me."
Emmerdale airs weeknights on ITV1.