If you are talking to a classically trained musician - it's really like an American talking to a Brit. It's the same language, but the nuances are different. There is no reason for them to know I-IV-V, that is a communication tool pertaining specifically to pop music. (When I say pop music, I mean rock, country, soul). It helps us communicate in a band situation to quickly put a song together. Whereas a classical musician is going to hand out sheet music to communicate the same thing. You won't hear them say, let's do Beethoven's sixth in G, it's starts on the I, then goes to the VI ... etc. The emotional/psychological underpinnings of music are also written on the sheet music for the classical musician. The way they interpret those directions is where the musician adds their own emotion/personal touch.

My sister is a classically trained violinist. She has no idea how to improvise, doesn't need to, but she understands the different scales/modes and how they are used to improvise. It is just a daunting task to her, she will literally break out in a sweat. Her husband is a classically trained cellist, he will break out into Led Zep on that cello at a moment's notice and will also take a lead. It is fantastic. He gets both genre's, likes it, practices it and is comfortable with it.

So, back to your question, what would you teach a new guitarist or bassist? If they just want to play in a band - I'd teach them I-IV-V stuff. If they want to be a studio musician, I'd tell them to learn how to read music, practice their scales and send them to GIT in LA

Agreed. Classical to pop is a completely different theory animal. I had to take both Jazz theory and traditional classical theory when I was studying music at UNT and I-IV-V is as foreign to classical players as a French/Italian 6th is to jazz. In classical theory, the "sus" concept falls into the realm of cadence (and false cadence). Classical theorists can analyze almost anything down to a substitution of a (plagal)IV-1 or (perfect)V-1 cadence.

As to music theory, well I'm actually pretty well trained (you could probably already tell from above). I can read music well, but am not a great sight reader. I'll need to run through a tune a few times to get it under my fingers and it takes longer as the number of flats and sharps increase. I know the majority of the traditional modes by heart but gravitate towards a few, not really comfortable with any of the harmonic minor modes (I know you said basic, sorry!).

Now, as to how much I apply while playing? Well that's weird. Typically I analyze the piece before hand looking at the changes and what chordal and scalar melodic ideas I might use. When I play, my mind is most often blank. In fact, I'm nearly always surprised when I listen to play back...

I find this sad as I have a theory that music IS THE language of emotion. Words alone cannot truly evoke emotion in another human (as in relay someone elses emotion as a communication method, not piss someone off or make them sad or whatever).