What are the effects of excess fiber, or abuse of laxatives on the digestive system?
There are 2 questions here, really. One about fiber, and the other about laxatives.
Personally, I'm a big fan of fiber, but if one is increasing the amount in the diet, it's a good idea to do it gradually, the same way one would make other dietary changes.
I'm not a dietician or nutritionist, But I can't imagine a person with a rich and varied diet consuming enough natural fiber to be harmful, providing the caveat above.
Generally, chemical laxatives depend on irritating the bowel, rather than filling it, and I'd recommend avoiding them, unless on doctor's orders.
If you're going to have some sort of medical exam or procedure involving the digestive system, you may be asked to take castor oil or some such laxative beforehand. That makes sense.