More like, find the chord the guitar or keys are playing, and play among the 1-3-5-7 (flatting or sharping where necessary).So it's figuring out the root to the chord, and playing the scale for it?
For example, if you're playing a song where the key for the song is Cm and where the verse is in Cm as well, then yes you can play a Cm scale over it and it will "fit". But most songs have a chord progression.
Lets take a quick look at What A Wonderful World (Ramones cover). The song is in F but there are lots of chords. So you can noodle around with an F scale but it won't sound right as the song moves along.
Here are the verse chords (assuming the tabs are correct, which is never a sure thing). For the measure where they are playing an F, the chord tones (you're safe zone) are F (root), A (the 3rd) , C (the 5th) and E (the 7th). Plus you can insert full scale riffs (F, G, A, Bb, C, D, E) and chromatic runs of course. But when the song goes to the next measure now you have to be in Am, meaning the chord tones are A (root), C (flat 3rd), E (the 5th), and G (the 7th). Et cetera for every chord change.
So an example of a line you could use for the first two measures might be F A C E G A C E.
A lot of bass players keep it simple by playing only the root, or the root and the 5th, and that's ok if it sounds good with the song. But try adding in the 3rd and 7th, and try making it flow. When you start playing a lot of notes the trick is making it sound like you're not starting over every time the chord changes. Listen to pretty much anything James Jamerson played. He kept it to chord tones and chromatic runs, he utilized a lot of "root - fifth" when needed, but he made the bass sound good.
| F | Am | Bb | Am |
| Gm | F | A7/Db | Dm |
You can also look up videos of Carol Kaye talking about the importance of learning to play chord tones. She's big on that.
Also, consider lessons. I only took a few and so I've had to learn as I go, and sometimes my ignorance shows. Hopefully it didn't in this explanation but if it did, someone will correct me and I'll learn from that as well. I would be a better player if I took lessons.
