Most of the time the bass in a band plays the root of the chord of the moment. In most genres, it is stylistically desirable for the bass to occasionally fill in the spots between strong beats with other notes, and we’re inclined to rate a bassist’s abilities based on how inventive/supportive these between-the-beats figurations are.
Regardless of genre, it’s not normal for the bass to be playing the melody along with the singer. It’s a distinctive sound when that happens, though. If you want to hear what it sounds like when bassist and singer are in melodic sync, listen to “Freewill” by Rush (Music: Geddy Lee, Alex Lifeson, Lyrics: Neal Peart).
Before I get into this tune, let me just say that even if you aren’t a fan of Rush or this particular genre of music, as I am, you just have to stand in awe of this band’s incredible musicianship, both as individuals and as an ensemble. Playing this tight doesn’t come via any avenue other than constant hard practice.
As you’ll hear, the verse melody is a two-part structure, where the first part (“There are those who think…“) gives us the melody in the vocal line, and copied in the guitar and bass. The second part of the verse (“A planet of playthings…“) starts similarly, with voice and bass in unison, but the vocal line quickly moves off and sings independent of the bass.
The chorus (“You can choose a ready guide…“) follows a similar pattern: the vocal line, guitar and bass are all in melodic and rhythmic unison for the first two lines, then independent of each other for the next two lines.
Sometimes the decisions regarding what the bass might do can be made at the recording stage, but given the complexity of music that is Rush’s trademark, I suspect that this was part of the songwriting process.
So the process of analyzing this song needs to include some discussion of the musical impact of this musical rarity: a bass playing the melody along with the singer. (In fact, in this situation, a case could be made that it’s the vocal line that copies the bass line, particularly in the chorus, where it sounds more like an animated bass line than a standard melody line.)
There are songs by other groups that do this, often to a lesser degree, like the vocal line at the start of the chorus of The Beach Boys’ “Good Vibrations” (Wilson/Love), which is largely similar to the bass line.
What does it contribute to the sound and feel of the music when bass and voice are performing the same notes? I think this kind of unison playing throws a more intense spotlight on the melody and the lyric of the moment. It’s a strengthening agent, musically speaking. We tend to sit up and take notice of what the lyric is giving us at those moments.
In essence, unisons between the bass and vocal line are a musical effect, and in that regard should be used with care, and definitely not overused. Like any musical effect, it can quickly lose its intended effect if it happens all the time.
But used judiciously and sparingly, unison between bass and melody can demand instantaneous attention from an audience, forcing them (in the best sense of the word) to listen to the words of the moment.
In that way, vocal/bass unisons can be one of many potential tricks up the sleeve of good songwriters and producers. It’s a technique that will work better in some genres (rock, prog rock, for example) than others. So as with all musical effects, you use the technique if you like what it does for your song.