Thanks for this comment. I totally forgot about this when I posted mine. You're understanding of soluble/insoluble matches mine although I think soluble does contribute to drawing water into the stool and giving it shape. Edit - Soluble fiber dissolves in water and forms a gel - insoluble absorbs water and provides a fibrous matrix. Both are involved in stool firming

Seems most supplements are soluble in nature. So maybe if you need to speed digestion or solidify loose stools you should focus on getting some insoluble fiber naturally rather than relying on supplements? Definitely seems like an important distinction