Well, if the kawarimi didn't look like you, at least for a little bit, would kinda defeat the purpose. Like in the first couple of episodes, Kakshi uses it when Sasuke threw shurikens at him. The illusion didn't go away until a couple of seconds after the log he was replaced with was hit. Probably acts like kage bunshin where a solid hit dispels the henge that was placed on the kawarimi. Same thing happened when Sakura used the jutsu when she was 'fighting' the sound 3 during the cuunin exam. The kawarimi didn't immediatly show itself after she was hit. Although I think in this case the monkeys were getting hit enough to dispel it almost immediatly. I woulda used kage bunshins to fool that ninja before I would use a henge on some dumb animal that won't even act like me. Then again, it is a filler.