This…this is the Holy Trinity!
This is going to be a long one because you are asking the right questions.
2 are from repeating practice and muscle memory, Accuracy and Fretting. Timing is from instinct, natural or artificial. It’s what makes the bass lines popped.
Fretting inaccuracy usually comes from undiscovered techniques, “Every” bass players struggled with this. 3 main issues and usually the answer to the problem.
First left hand posture, look up Josh’s hand left hand lessons.
Second, look at the fret where you want to end up not the travel distance. If you are traveling from 2nd to the 8th fret, don’t look at how just where. just look up to where the destination fret and be there don’t smoothly move your hand, just be there. Which brings me to the last common mistake.
Third, forget smoothness when you move your left hand up and down the neck, think “Warp speed”. If you hand needs to go from 2nd fret to 8th fret don’t travel up the neck warp it down and be there ready to plug 8th fret.
I hope you get the picture.
It really depends on the song, I’d approach technical songs in 2 ways.
First, break it down into sections. Intro/ Verse/ Chorus/ (Break)/ Verse and Outtro. Usually technical bits would come from the main Groove, somewhere in the middle and at the transition to the next section.
Once I get the first 3 then I just rinse and repeat until I got it down and get ready for the more difficult part which usually towards the end.
I’ll give you a good example of a song, it would answer many interesting aspect of what @Whying_Dutchman , @faydout and what @sshoihet was suggesting.
This is a live version of Change the World, by Clapton. Nathan East on bass. Intro is well quite technical, each Verse and Chorus did not repeat they are different, and the outtro is just CRAZY!
I skipped the intro when I started, because there were skill elements that I have not yet acquired, then work on the first Verse and Chorus, I repeat that all the way til the Outtro. Once I got that down then I start the 2nd set of Verse and Chorus and then 3rd. Since the Outtro is easier than Intro I worked on that before the Intro.
When I got them all down, then the real hard work began. I’d listened to the true notes value and articulations pretty much the spunky attitude of the song. Replicating this can take some time.
This cover I played on a Short scale Multi-scale that stretches all the way for visual effect so fretting is not as intuitive as the normal MS. It usually takes about 5 minutes to recalibrate for me.
If you make it all the way down here I hope that this would help and enjoy my hard earned cover. 
It took me a while to get good at playing in frets above 8 because i didn’t play there as often, especially frets higher than 12. Right now i’m working on playing more solos in the upper frets so i’m more used to moving there more quickly
