When not eating I would not eat or drink anything that has any calories in it. I typically drink water, black coffee or green tea.
I have also tried longer fasts from time to time (2-7 days typically, 2 weeks once) and then I would supplement with vitamins, l-tryptophan and lean broth/bone broth in moderate quantities (a cup a day), just for safety and general well-being (l-tryptophan is precursor to serotonin and mildly anti-depressant).
Intermittent fasting is hard at the beginning but after about 2 weeks I get used to new regime. It seems it is the same every time I start it anew. It might be getting easier but I think that's because I already know what to expect.
Also, when intermittent fasting it is much easier to start longer fast. I find, when fasting for more than 2 days first two days to be the hardest.
I found I can't just decide never again to eat the things I like and never again feel sated. Feeling hungry constantly isn't exactly my idea for the rest of my life.
Alternate day fasting lets me survive a day with no calories knowing that the next day I can be sated and eat (almost) whatever I want.
I have also found that when I eat every other day I put much more attention to what I eat on those days -- I mean, if I just did not eat yesterday and I won't eat tomorrow I want to eat well today. Even if I am going to eat sweets -- I will try to go for something better and not feed myself with garbage.
I also found that alternate day fasting is good willpower training. Being able to restrict myself from eating for an entire day somehow trains me to be better at other things that require willpower.