WordNet sense 1 (not subject to the control of the will):

nonvoluntary, unvoluntary

WordNet sense 2 (made necessary by an unexpected situation or emergency):

unexpected

WordNet sense 3 (not scheduled or not on a regular schedule):

unscheduled

WordNet sense 4 (lacking spontaneity; not natural):

constrained, strained

WordNet sense 5 (speaking or behaving in an artificial way to make an impression):

affected, unnatural