Right now I am recording an album with a new band that is a vocal oriented project. The singer also plays guitar and writes in more of a singer/songwriter waym which means basically writing lyrics and strumming chords until he finds the right ones to fit the vocal. He doesn't know music theory but that's OK and I will tell you why.
Theory does help you understand music better and helps you communicate better, but oftentimes it will stifle the creative process because now you have a set of "rules" to think about.
A good example is, let's say we have a song that has a chord progression of Am, C, G, D and F. If you understand how chords are formed and how diatonic harmony works you will know that a D chord is spelled D, F#, A and an F chord is F, A, C so if you play an F# over an F chord it will clash and if you play an F over a D chord it will also clash. Keep in mind this music is much more commercial than my other band No Walls and the intended audience might not have as tweeked of an ear as an instrumental audience. So what to do?
I know some players would want to change the D to Dm and that would keep everything in the same key, But the most important part is that it sounds better with the D major chord. One way to approach it would be like a jazz player and play the chord tones on the D chord. I tried that and it sounds boring and contrived. What I have worked out is to play a combination of E Pentatonic minor(E, G, A, B, D) and A pentatonic minor (A, C, D, E, G). If you combine these two scales you get (A, B, C, D, E, G) and it sounds really cool. You can also hang on an F when the F chord comes up (Just like Jimmy Page did in "Stairway To Heaven") and it will sound great. So now I have a brand new scale to work with!
To sum it all up, don't be afraid to make or break your own "rules" to fit a situation that's how music progresses and develops. I mean somebody had to add a flat 5th to a pentatonic minor scale, and now everybody uses that.
Mike O'Malley's instrumental power trio is called No Walls and their latest CD is entitled "World Abroad". He has been playing guitar for almost 30 years and graduated from Music Tech in Minneapolis in 1988.
O'Malley currently has 45 guitar students that range in all ability levels and styles.