During a hostile act the most important thing to do is to remain calm. The wrong move can get you injured or killed.
If possible, you should try to call the police (you don't have to say anything to them, just call). If any emergency officials give you directions, follow them.
-Don't make any sudden moves. (Moving quickly is dangerous in this situation unless directed to do so)
-Try to hide (if there is a place to hide, and if it's safe to move).
-Remain quiet (to prevent provoking the person who is holding you hostage).
-Be cooperative. (This could save your life)