A very common musical concept to keep music interesting is to use complementary rhythm. This means, when for example your main melody comes to a rest (e.g. holding for a whole note), you bring in another side line/figure/motif somewhere else. Having a look at the very popular film themes, you will very often find a woodwind run, a horn motiv etc. as soon as the melody holds a long note.
This is very helpful to keep the energy of the music up as a simple held note for several counts will just create a massive drop in energy and rhythmical momentum which you can counteract with a complementary rhythm somewhere else.