Lots of good advice already. Good "Feel" is the key. Doesn't require a lot of technique most of the time. The right feel for the song requires really listening to the song, being a good mimic. You need good time, as much of the groove is playing stuff around the beat, sometimes before, sometimes after, often not directly on the beat. You may be laying back, or anticipating, but either way you must be able to do so without altering the timing. Anticipate without rushing, lay back without slowing down. Pay attention to the articulation. How long do the notes ring? Are there accents?
You have to be able to play the seemingly simple lines that are out there and make them come alive. Anyone can play them "straight", make them sound boring. I pretty much hate Bob Seger. Simple predictable boring songs. I went to see him in concert because a friend of mine was playing in the opening act. I was tempted to leave after they played, just skip Seger altogether. I figured I'd stay for a few songs. Amazingly enough, they played those simple boring predictable songs with a lot of energy, great feel. Maybe it was playing for a hometown crowd, but they played their asses off. I was very much impressed. Learned a lesson that day.
There aren't a lot of technically challenging basslines in most commercial music. It's not a matter of technique in most cases, its a matter of restraint, a matter of NOT trying to show off your technique every chance you get, but focusing on the drums and locking with them, while simultaneously listening to the rest of the band and matching what they're doing also. You're the bridge between the rhythm and the chordal structure. You must match the drums rhythmically, and the music harmonically.
The general rule is, it's what you DON'T play that gets you hired. Playing too much all the time gets you fired.
I've been playing with a really good band lately, and I've been playing pretty conservatively, concentrating on just keeping up, playing with a lot of feel, but following the original bass lines pretty faithfully. Was worried at first I should be jazzing it up more, as its a real high energy act. Yet I have had a lot of people coming up and complimenting me on really holding the bass down. I'm actually amazed, it's been pretty much every gig we do. And that's one thing, but I always figure maybe people are just being polite. One that really stands out for me was the bass player from a real live pro band said I was a really good bass player. What makes it special for me wasn't that he's a pro bass player. What makes it special is he didn't tell me, he said it to someone else, who passed it on to me. He had no reason to lie to be polite.
I'm no technician, I'm reasonably competent, but plenty of guys can play circles around me. I try to play with a lot of energy, not a lot of notes.
The key is this: It's not how many notes you can play, its how you play the notes you choose to play.
The other key is this: Don't be a jerk. Show up, on time, know your parts, have decent equipment that is up to the task at hand, don't be a prima donna. The guys that can play rings around you and know it think people have to kiss their butt to deserve their participation. You don't need a bass player that can play like lightening, they may get the job for a short while until they stab themselves in the back with their attitude. There are plenty of really beautiful girls out there that are really high maintenance and not worth the drama they bring to the relationship. Same goes with bass players, and really any other instrument as well. The trouble with bass players is that you don't really NEED a great technical bass player to play 99% of the cover songs out there, so there's even less reason for anyone to put up with drama than other members.
Randy