My daughter was very afraid of pooping on the toilet.
I have a potty in the bathroom and one day when I KNEW she needed to go, I ran her to the bathroom and sat on the toilet while she sat on the potty. She was VERY upset and crying and telling me NOOOOO! So, I told her that we were going to both push the poopie out together. Then I made all sort of over exaggerated noises as I "pushed the poopie out." Next thing I knew she was yelling and grunting and making a bunch a noise too as we both pushed it out. Once she did it, she immediately jumped up and congratulated herself and I did the same w/ lots of horray's and good jobs, etc.
After that first hurdle, she was okay w/ the potty and bowel movements. To this day she still grunts and tells me that she is pushing the poopie out.
One other thing, I told her to tell that poopie to "Get out of there!" That's one of the things I used to encourage her to push it out.
I think all the noise etc. that I made distracted her from her fear a little which allowed her to relax just a little to make the bowel movement imminent whether she wanted to release or not.
I'm really big on the princess saving herself!