oh it's very unhealthy, stresses your kidneys, heart, causee just the worst headaches

Everyone at a bodybuilding competition does this as well. It certainly doesn't seem good.

It is unhealthy and as far as I can tell no actor is a fan of it

The actor of Green Arrow talked about. The Green Arrow series has some really dedicated scenes.

A healthy, normal-weight adult can fast safely for a week, after that you should get a doctor involved to take blood levels to be on the safe side and take whatever micro-nutrients you get prescribed. The longest fast ever was done by Angus Barbieri, 382 days, on a diet of coffee, tea, sparkling water and supplements (as per doctor's order). He was an absolute unit going in normal weight people don't last that long.

Going without water will be deadly in three days or less, no exceptions. Doing it in a climate where you're not sweating helps but ultimately you have metabolic products to flush out of your system which won't work without water intake. The body is going to allow poisoning itself before it shuts down, that's the kind of situation where no matter what you intend to do, at some point you're going to catch a rabbit with your bare hands, rip of its head, and drink the blood.

Oh, and while I'm at it: Distilled water is safe to drink. Yes, it's easier to get water poisoning with distilled water as opposed to ordinary but either is an achievement and requires fasting as well at not giving in to that sudden urge to murder your salt shaker. We get the overwhelming majority of our minerals from food, not drinks.