As Sir Brian said, "the clothing makes the man", and I feel that in some cases, it amplifies who they already are. I'll give a few examples, as I'm having a hard time phrasing it well.
Take the store owner in Houston who killed 3 robbers when they threatened him and his wife. Or the Massachusetts governor who pulled kids from a burning van. Or the police sergeant who bought a flag to drape over a retired firefighter who died and had no family left to do it. I think all of those are knightly acts, and to put those men in "knight's clothing" would reinforce who they already are, but their actions have defined them more than any clothing can.
At work, I have to wear nice shoes, dress pants, dress casual shirts. I wear it, but it's not me, and I'm not always comfortable in it. I'm a jeans (or shorts) and t-shirt person. When I'm "dressed down", I feel like myself ... relaxed and practical. When I'm "dressed up" for work, I feel like I'm "playing a part" for work, since I'm doing something I normally would not. I think the same would happen if you put someone in a knightly kit who shouldn't be ... they are able to wear it, but it's not "them".
Since heroes (and knights) look like normal people, appearances often make the first impression ... but after that, deeds make the person. I think that is a part of what living up to the knightly ordeals is; dress the part, act the part, be the part.