Behaviour is a characterising attribute. For example, "the behaviour of the monkeys is peaceful". While, an act refers to a single instance in time. The act of doing good, the act of speaking english. So, for your case, it is better to say that "Apologizing is a brave act". However, if you were studying a group of primates and you noticed the apologized, you might describe this action of theirs as a behaviour, and so may say that "Apologizing seems to be a brave behaviour among them".

Act = performance Behaviour = manorism Hope this makes sense!!

Act is a verb. Behavior is a noun. Hope this will help you.