My home-grown theory is that, if you know what key you are in, you will/should know what the notes of the scale are. Example, Look at Us, let's say it is in C, for ease of recognising the notes. No sharps or flats to worry about. At the end of the verses, there's a bridge. The words are "In a hundred years from now..."
The chords are C, at the end of the previous line, stepping down to G7 - although G works just fine for the example. Question: as the singer draws breath, how do we walk down from C to G? The "walk down" sounds good if you do a C at 8, no pedals, then a Dm at 6, E's lowered, a C at 3, A&B pedals in (squeeze them in for effect), then a G at 3, no pedals. Use strings 3 & 5 throughout. Sounds ok, but what is happening musically?
The top note in the C chord we start with is an E, the third note of the C scale. Then we go down from there, using the notes of the C scale. The next note down is D, but in the C scale that's a minor chord, so it has to be Dm. Then from D down to C, the "home" note of the C chord. Finally, back down one from the C to a B, which is the seventh note of the C scale - scales fold back on themselves, once you get to the 8th note it's the same as the 1st note, only an octave higher. ("Octave" - the clue is in the name.)

Although in theory the chord for that note should be a Bdim, in practice the dominant seventh - the V chord - works extremely well and is easier to find for most of us.
So we have gone 3 - 2 - 1 - 7, or C, Dm, C, G7 - or just G, because we want a nice "solid" sounding chord there to start our slide back up again, and a 7th always sounds unresolved or unbalanced, as if it wants to go somewhere. In "Nashville Numbers" it's I - ii - I - V. It's knowing how to find and construct these little scale passages that constitutes much of the bread-and-butter work a steel player does in supporting the vocalist and the rest of the band.
The ideas aren't hard, if you can just get happy with the idea of how a scale is built. The whole thing about why sometimes a chord has to be minor, not major, is all about the gaps between the notes, and would be the subject of a talk on its own.
All this works fine unless you want some unusual tonal shadings for effect, then you have to use notes that don't "belong" in that key, and chords that are "foreigners" too. That's when you have to branch out a bit.
