I think most of this has already been said, but here's my tuppence worth, in no particular order:
Most of the time don't worry about being emotional; worry about being in time with your drummer.
For a song with room for emotive bass, consider a fretless.
Play less notes, and let each note breathe.
Do something with each note; slide, bend, slur or add vibrato.
Don't do the same thing on every note, or let your vibrato become syrupy.
Play it like you're singing it.
Listen to the vocals and try and play a counter melody; think of it as a duet with your singist.
Don't be overbearing; enhance the singer's performance, don't overwhelm them.
Use dynamics; be quiet in some places and louder in others. A slow build/crescendo through a song to its peak can work wonders.
Less is more.