for songs, a really powerful and effective thing you can do is to make complementing the vocals the focus of your own fills & melodic deviations from the written notes... 2 or 3 of the right notes placed to echo, counterpoint, harmonize or even sound in unison with a vocal phrase can be infinitely more effective than 15-16 notes out of context
it's an old old cliche but if you use your ears and react to what's going on around you, the notes will usually take care of themselves...