Writing to a bassline
 

Hi, I'll get right into it. How do you write interesting guitar parts to an already put together bass line? Do you simply repeat what the bass is playing? Obviously the bass generally plays root notes to chords, but what about song that don't follow that same technique? Bands like tool and Primus don't tend to have the bass play with chords, so what am I missing? Thanks

A bass line can also be thought of as a rhythm track and as a melody line, as if it was a regular guitar itself. You could write the guitar to sometimes base itself on the root note of the bass, sometimes play around it, sometimes sticking with the same rhythm, sometimes playing counter. As long as it sounds cool, anything goes.

What do you mean by "playing counter"?

Opposite. Or differing from the original rhythm but still sounding good.

Just work in the key and scale and keep the changes properly timed and everything will be fine. If there are certain parts that seem more pronounced than the rest of the bass line, say the bass player uses a chord or something like that, you can either write toward the sound of it to embellish it, or above the sound of it to contrast it, depending on which way you want to go with the sound. Drums can also be used to your advantage with guitar depending on what the drummer is doing and what the bass player is doing, you could work around it as well.

One of my songs was written with a minor pentatonic descending scale bass line. It offered a counter motif that when placed together with the piano, created an entirely different feel. Some would just look for the root notes on measure and take it from there.

Add drums and see if that helps

Like a jazz vocalist taught me, bass goes with the kick.
But, there are many exceptions to that rule.
Bass can be used as a counter melody. Also as an add on to what is being played.
Some chromatic runs. Walking bass lines.
See: the instrumental track Red Baron.