Dr.GastroMayo said:

I added fiber, made it noticeably worse, and only later worked out that I had added the bulk without adding the water.

That is correct as far as it goes, and here is where it stops going. The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work. Less food means less bulk, slower motility means more water reabsorbed, and the result is a smaller, drier, slower stool. Insoluble fiber adds bulk to a system that is already moving slowly, which is why it frequently makes things worse. Soluble fiber holds water and is the better first choice, and an osmotic agent works with the mechanism rather than against it.

Dr.GastroMayo said:

I added fiber, made it noticeably worse, and only later worked out that I had added the bulk without adding the water.

Worth saying plainly that not all of this is benign. Severe constipation on delayed gastric emptying has produced obstruction and ileus cases, rarely but genuinely. Pain with vomiting and no bowel movement is not a fiber conversation.

This one has a reasonably settled answer, so here it is. The escalation order that reflects the mechanism: water and salt first, then soluble fiber, then magnesium or an osmotic agent, then a stimulant only briefly and only if the osmotic has failed. Stimulants first is the commonest error and the one most likely to end in a cycle of dependence.

BethLabQueen said:

The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work.

Can confirm. Same sequence, different timescale.