I'm never going to tell some of my friends what I've been up to.
Why don't you want your friends to know what you have been up to?
It's just something I feel I have the right to reserve.
Well, everyone has secrets and it is your right to withhold information that you don't want to share. If anyone tries to pry or force you to tell them stuff about you then that friend is rude and disrespectful and should be kicked out of your friend circle.
I just want to see how they react.
There are things that everyone wants to keep private. It's important to maintain boundaries in friendships so you are not sharing more of yourself than you are comfortable with.