On the contrary, quite often it's OK to play something unpredictable which would show our own input and interpretation.
One way to do this is to treat music like code: first we have to deconstruct musical text and discover intervals and chords which fall on the beats of the measure. Then we can take out some of the composer's passages which fill in those intervals and chords and change into our own - the skeleton remains but musical texture is already different. It's very much like paraphrasing technique used in literature.
Treat composition like model which can be developed even further (most composers did the same with the pieces of other masters).