This study was designed to investigate the effect of student performance level on teachers' expressive-emotional discourse in musical instrument classes, the possible loci of emotions present in teachers' discourse and the existence of a tradition of expressive factors in the interpretation of certain musical works. Forty-seven guitar master classes were recorded, and students' performance levels were subsequently evaluated by independent judges. All material was then transcribed and coded according to the following variables: emotion words, emotional analogies and teaching of expressivity. A questionnaire administered to professional guitarists elicited information on the expressive tradition of the works played in these master classes. Statistical techniques (analysis of variance [ANOVA], the Mann--Whitney U test and the chi-square test) were used to analyse the variables. Results suggest that students' performance levels and the expressive tradition of the repertoire played can be decisive in how teachers construct their expressive-emotional discourse. Finally, as a teaching strategy for musical expressivity, focus can be directed either towards the emotions felt by the student (i.e., internal locus), or emotions expressed by the music or another source external to the student (i.e., external locus).