someone did some improv solos in cmaj scale while the rest of the song is being played in key of e minor. Why does this work? Saw a guy do it last night and he had no explanation either... Thanks
First, notice that the key signatures of E minor and C major differ in only one note: E minor contains an F♯, while C major contains no sharps or flats. Since these two keys contain almost the same notes, it should come as no surprise that a C major scale should work when playing in E minor.
Another way to look at this is to notice that the third mode of the C major scale is E Phrygian. This mode contains the same notes as an E natural minor scale with a ♭2. In this view, E Phrygian is just a colorful choice to play over E minor.