A song in the key of G

You could over some chords but not all. That progression isn't strictly diatonic due to the F, F# and B (I assume those are major chords).
My advice is to use your ear as your #1 guide with knowledge of the chords and their movement - as opposed to strict diatonic theory.
I'd also advise against abandoning the root too much. Root movement with passing tones can create great bass lines. So much of a great bass part is in the rhythm, more-so than elaborate harmony IMO.

 

What style of music? Is there a lyric/melody already?

Just another observation, if the B in the last line is major, then it's not diatonic either. In G it would be a Bm.
As for your question, I suppose you could, keeping the non-diatonic chords in mind. However IMHO using chord tones and passing tones generally works better but that could be dependent on the type of song.

 

For a bass line you need to be outlining the chord movement and signposting this - you cannot just take random notes from G Major and use them without reference to the chords passing by!

It might help you to know which chords are diatonic, as has been said - but basically you have to being saying something about the chords and connecting them together!

Of course you don't have to play all roots every time - so a D on the G chord will be quite strong - chord tones are OK, although beware of making the rest of the band think they are in the wrong place!!

 

Never let the pentatonic scales (major and minor) get too far from your hands or your imagination. Make peace with flat 3's and flat 7's. Keys don't have to be rigid, they can accept other notes and chords. The more crayons you have, the prettier the picture.

 

As asked before, what kind of music is this? Generally, you'll want to play in certain ways to fit the music best.

Also, is each chord a different measure? Of is there a point where two chords share a measure? The way the chords are placed can tell you how to accentuate them.

 

Picasso's blue period would disagree with you

But on topic, being melodic always is a good idea, the thing is though thoguh someone has to be holding it all down or everyone will be lost. try whatever you want though, it's your song and you should push whatever boundries you feel so inspired to push.

However, be prepared for your band not to get what you're trying to do.

Also, when you are in the non diatonic chords (F, F#, and B) you need to adjust your note choice accordingly

 

Hitch hiking on the last post. Melody works, however, is your job to provide melody or is your function in the group to provide a steady beat and accent the chord movement?

I play rhythm guitar in one band and bass in another. In the first band our bass and the lead guitar have through the years worked out how each can provide melodic licks and not step on each others toes. That takes a pretty good bass player.

When I'm the bass, I still have to stick with the basic progression and R-5 or R-3-5-3 riffs.

My point - melody is great if you are not stepping on some one's toes or neglecting the beat.

 

First off, i'm basically in the stick-to-the-roots camp. There is a 90% chance that the rest of the musicians expect this, and it really helps provide a rock solid foundation for the music. Basically the approach is root on the down beat, chord tones on strong beats, and passing tones other wise. Melody can be achieved in how you choose the "in between the roots" notes. And of course sometimes, just pedaling on the root is actually the best choice.

BUT

it's good skill building to analyze the chords for alternate approaches. Here's what I will often do, taking the first 4 chords (G-D7-C-G). i'll write out a matrix of all the chord tones stacked vertically:

Code:

G D7 C G
B F# E B
D A  G D
  C

and then start literally connecting the dots to find new ideas. Judge the results by how the rest of the band's parts sound against it. Usually for my situation (original indie rock) this approach doesn't yield something that improves over just root to root motion, but sometimes nice stuff results.

 

Excellent advice.
Because Mambo's pattern shows the possibility to play G -F# - E - D over those first four chords. Does it work with the melody?

Just noticed something important.
Mambo says: Judge the results by how the rest of the band's parts sound against it. That's very sound advice. However: the classic order of harmonic importance is 1. melody 2. bass 3. the rest. So maybe the others should change something to make room for the bass part.

Needless to say finding passing notes is my passion.