I've seen the same "I'm not afraid" attitude in many of the riding students here. They try to say the right things to show that they aren't afraid of the horse, but their body language says otherwise.
The one thing that seems to set people off the most relates to what ever accident they had to make them fear horses, or size for those who have no experience around them. As an example, a lady who fell from a horse, broke multiple bones, and lost an eye becomes almost paralyzed with fear when her horse makes any sudden move under saddle (like lifting it's head, cocking a hip, and other such things that I consider to be minor). She has no problem relating to the worst behaved horse on the ground, though. Many of my teen students get very nervous when dealing with loose horses that are puppy dog friendly, such as in a stall or paddock. The horse walks right up to the person, invading her space, and often puts those oh so scarry feet or that monsterous mouth just a bit too close.
Ironically, the foals do not bring the same fear response as older horses, even in cases where the foal is of similar size (e.g. draft foal compared to a pony). One mother who comes by started out terrified of horses, but after playing with foals, and watching them grow up, she's only "a bit nervous" with the older ones.
Hope that helps with your character developement.
