It's been said elsewhere here, but I thought I'd reiterate it since it chiefly earned me my fissure(s) -
If your BMs are regular enough, insoluble fiber can make things worse for you. It's more to poop out, wider, and harder to do at that.
My doctor, despite me already eating unruly amounts of fiber and eating unconditionally 'healthy' from earning a portion of my living as a fitness professional, advised I pile on more to help with hard stools. No discriminating soluble or insoluble, just unrelating praise for more fiber and a perscription laxative despite my being already regular. I was at what I thought was wit's end, so I complied. That made things 100x worse by causing my first experience with day-long spasms, either tearing my fissure more or granting me a new one.
My chief strategy that's helped has eating less of that at it's core, along with more fatty oils and a recent addition of Milk of Magnesia. Everything affects everyone differently, and more fiber indeed helps some that don't get much at all. But those that are already regular, eat well and exercise daily be warned - like me, more fiber may be your undoing.