Originally Posted by: NickFerra
https://m.youtube.com/watch?v=ZQR2WiaGGTc

OK, this is the one with the chord prog: Em, C, G, and A

Your guitarist is doing something like this:

|-----------------------------------------------------------|
|-----------------------------------------------------------|
|----------15-15(B)16-15-15(B)16---------15-15(B)16(B)15-12-|
|------14-----------------------------14--------------------|
|--14------------------------------14-----------------------|
|-----------------------------------------------------------|

|---------------------------------------------------12------|
|---------------------------------------------------15(B)---|
|----------15-15(B)16-15-15(B)16---------15-15(B)-----------|
|------14-----------------------------14--------------------|
|--14------------------------------14-----------------------|
|-----------------------------------------------------------|

So, he's outlining the E minor chord but adding a flat 5th. It sounds like the C in the beginning is actually a C7, which fits nicely together, because the B-flat is the 7th of the C chord.

But it's mostly just E minor with a flat 5th. Or E pentatonic minor with the flat 5th.

The vocalist is singing:

|--------------------------------------------|
|--------------------------------------------|
|-----4-4-5--5-5-4-4-------------------------|
|--2--------------------5--------------------|
|--------------------------------------------|
|--------------------------------------------|

And that's straight E minor scale. So that would work also. The vocalist is usually carrying the melody & you can't go wrong just playing what he's singing.

Hope that helps! Work on those improvisation tutorials & you'll get the idea of how this stuff works.