For diarrhea, soluble fiber would be best to focus on vs. insoluble fiber. This will help slow the movement of foods through the digestive tract.
Furthermore, the timing of fiber intake can play a role in managing symptoms. For people prone to urgent, loose bowels in the morning specifically, limiting insoluble-rich foods like salads at dinner and including a soluble-rich veggie (squash, skinless sweet potato), cooked grain (rice, pearled barley, quinoa) and/or soluble fiber supplement (Citrucel, Benefiber) can help smoothe out choppy morning bowel patterns.
If symptoms tend to occur in the afternoon or evening, having a super-soluble breakfast (like oatmeal, chia seeds and banana) can help keep things flowing smoothly later on in the day.