The bass player has the most crucial role in the band. Everyone in the group depends on the bassist's subtle (and sometimes not-so-subtle) lead. If the guitarist or saxophonist makes a mistake, hardly anyone will notice, but if the bassist makes a mistake, everyone in the band and the audience will instantly know that something is wrong.
Making the link between harmony and rhythm
You're responsible for linking the harmony (chords) of a song with a distinctive rhythm (groove). This link contributes to the feel, or style, of the music. Feel or style determines whether a song is rock, jazz, Latin, or anything else. You want to be able to emulate any bassist in any style and, at the same time, be creative — using your own notes and ideas!
Moving the song along
Every song is made up of chords that are special to that tune, and all the notes in the tune relate to the sounds of those chords. In some songs, all the chords are the same, and so all the notes relate to that one chord sound, making such songs easy to play. Most songs, however, have different kinds of chords in them; in these, the first group of notes in the tune relates to the first chord and has one kind of sound; the next group of notes relates to another chord sound; and so on throughout the song.
By playing one note at a time in a rhythmic fashion, you propel the music along. You set up each chord for the other players in your band by choosing notes that lead smoothly from one chord sound to the next.
Good music creates a little tension, which then leads to a satisfying release of that tension (a resolution). For example, you can feel the tension and release in as simple a tune as "Twinkle, Twinkle, Little Star." The tension builds as you sing the first line: "Twinkle, twinkle, little star." Can you end the song right there? No, because you want to hear how it ends. That's the tension. When you get through singing "How I wonder what you are," you feel a resolution to the tension, a sense of coming home. You can end the song there; in fact, that's how it does end. The bassist plays an important role in creating and releasing tension. You're pretty much in the driver's seat!
Keeping a steady rhythm, or a pulse, is one of the bassist's primary functions. This function is called locking in with the drummer, because you work very closely with the drummer to establish the rhythm. So be nice to your drummers. Listen to them carefully and know them well.
Nothing works better than a metronome at helping you develop an unfailing sense of time. The steady (and sometimes infuriating) click that emanates from it provides an ideal backdrop for your own note placement, be it on or off the beat.
As a bassist, you need to have a very clear understanding of exactly how the rhythm relates to the beat. You need to know where to place the notes for the groove in relation to the beat. And you want to make your grooves memorable. If you can't remember them, no one else will be able to either — including the listener (who, of course, makes the trip to hear you play).
While the guitarists move through their aerobic exercises, dripping with sweat and smashing their guitars, you get to be cool. You can join in with their antics if you want. But have you ever seen footage of The Who? John Entwistle was cool. And, if you ever get a chance to see U2, check out their bassist Adam Clayton. He's one cool cucumber, too. Great bassists are just too busy creating fabulous bass lines to join in the antics of their band mates.