Like if you’re constipated and you take Miralax or Metamucil or whatever, do you just wind up with diarrhea? Or does it just cause you to have a normal, not-uncomfortable poop (albeit a rather large one)?
Firmer turds. But too much fiber suppliments can make it feel like you are flossing your inards with a nautical rope.
If your normal diet isn’t supplying sufficient fiber to keep the colon train moving on schedule then taking fiber supplements should restore normal pooping habits - that is, normal timing, consistency, etc. It restores the system to a normal state.
Some medications inhibit the “trains” from moving, in which case a laxative will likewise restore the situation to normal or a decent approximation.
Excessive use, over use, and/or mis-use can result in problems, including diarrhea.
In general, either increasing your dietary fiber or using a fiber supplement is least problematic of the alternatives for constipation.
Getting the dosage right is critical. There is no way to know the correct dosage in advance. The dosage listed on the package is never the right dosage for you, at this moment. Worse, each type of fiber has a different correct dosage. And what you eat has an effect on the right dosage.
Three words: trial and error. Three more words: don’t give up.
Two more words: go slow.
Changing the amount of dietary fiber or doses of supplementation too abruptly can lead to achy bellies and promises never to do that again.
I currently take three doses of psyllium fiber (mixture of soluble and insoluble fiber) and three doses of fiber gummies (insoluble fiber) per day. That’s 27-30 grams (depending on the brand of gummie) of fiber, plus a diet fairly high in plant fiber. But it took me nearly a month to ramp up to that without discomfort.
The bulk fiber stuff is safe, assuming you make sure to drink plenty of water with it. I suggest a heaping tablespoon in a 8 oz glass of water every day. Get some salads, fruit etc into your diet and make sure your breakfast cereal contains a decent amount of fiber.
Ad some probiotics to your diet also.
Stay away from other laxative types unless your MD sez so.
Careful - there’s sugar containing and sugar free. The stuff w/ sugar is dosed by a rounded tablespoon. The sugar free stuff has the same amount of fiber in a rounded teaspoon. Mistaking one for the other is a bellyache in a glass. Ask me how I realized this. 
Better still, mix the bulk fiber stuff into a glass of fruit juice! (Assuming you are OK with the amount of sugar there.)
I used to use Metamucil regularly, which I took with a heaping (I’m guessing about 16 oz) glass of prune juice. Eventually I quit that and then discovered I didn’t really need it.
To this day, however, I drink prune juice often. The nutritional label seems impressive: It claims to have all kinds of goodies: Potassium (12% / serving), riboflavin (15%), B6 (15%), niacin (10%), with a touch of phosphorus, zinc, magnesium, manganese, copper, uranium, and unobtanium. Even Gatorade isn’t even close.
That’s some pretty thick prune juice. 
Also, fiber can help avoid constipation in the future, but it’s too late to help this current bout of constipation. It could even make things worse.
No, I don’t agree with this. Resolving constipation is an FDA approved use of bulk-forming laxatives, and it’s a first-line recommendation (that is, before stool softeners and stimulant laxatives, try bulk-forming laxatives). It doesn’t work as quickly as a stimulant laxative - give it 2-3 days before you conclude it hasn’t worked - but it’s also not as painful as a stimulant laxative can be. It’s, as always, important to drink a lot of water when using a bulk laxative, and if “uncomfortable” becomes “pain”, then you should seek a doctor’s attention, but that’s true of constipation in general.
No? Hmm I must have misunderstood somewhere.