First of all what kind of diarrhea it is? how many times a day? Is it watery or bloody? What was your diet prior to diarrhea? Best test: stool test. Best solution: oral rehydration solution -ORS is the best option for any kind of diarrhea. Drink plenty of it to prevent from dehydration. Medically, fiber is never a good choice for any diarrhea until your stool test done.
It depends on what's causing the diarrhea. If you have irritable bowel syndrome, then a gradual introduction of fiber to your diet will definitely help, in the long run. But if its caused by a temporary situation such as food poisoning or a flu, then fiber is a very bad idea. Try this: stir a tablespoonful of plain, ordinary flour into a half-cup of cold water, and then add a teaspoonful of sugar and a few drops of vanilla (to make it more palatable). Drink this mixture, and repeat every hour or two, until you resume normal elimination. It simply absorbs the excess fluid and helps to restore your intestinal equilibrium.