I'm rotating an object in $3D$ space with respect to a relative base, or reference frame. I'm using a normal vector to represent the rotation angles. Suppose you have an object parallel to the reference frame; in this case the object has the following normal vector: $v=[0, 0, 1]$. So, the normal vector makes $90^\circ$ with x-axis and y-axis and $0^\circ$ with z-axis. (I suppose z-axis is the principle axis).
My question is: If we rotate the object $30^\circ$ around x-axis; what is the direction normal and the angles? is the change only on the angle around x-axis or z-axis also. I noted when I rotate $30^\circ$ around x-axis the z-axis also rotated with the same amount. This confused me. Any Help!.