#1
So lets say the bass and rhythm guitar are doing a simple progression in the key of A like A-D-A-E. The scale I (the lead guitarist) am using is the A major scale. Am I only able to use certain notes out of that scale depending on what chord the song is in or can I just use any note out of that scale since all the songs chords are in A major.

P.S. don't give me a hard time
Quote by beadhangingOne
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#2
You could use any note you wanted to from A major but in the interests of sounding good your best bet is to target chord tones (The notes of the chord you are soloing over)

Usually you aim to hit chord tones on the strong beats and potentially awkward notes on the weak beats
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#3
Simply, yes you can.

What I found helped a load when I first started with this was to make a tab on Guitar Pro that played a rhythm, while I played a solo over the top of it. It helps you get to grips with it.
#4
Chord tones will sound nice, but use non-chord tones for more dissonance and tension that you can resolve. Hell, take it a step further: You don't even have to use scale tones. Use outside tones. Experiment. Playing an entire song (and the song after that, and the one after that, etc. etc.) in a single scale and not straying at all gets stale real fast. Don't forget to have fun and don't worry about a note you think sounds bad... The answer is only a fret away.