Participation in the Liturgy
In liturgical celebrations each person, minister or layman, who has an office to perform, should do all of, but only, those parts which pertain to his office by the nature of the lite and the principles of liturgy.
Servers, lectors, commentators, and members of the choir also exercise a genuine liturgical function. They ought, therefore, to discharge their office with the sincere piety and decorum demanded by so exalted a ministry and rightly expected of them by God's people.
Consequently, they must all be deeply imbued with the spirit of the liturgy, each in his own measure, and they must be trained to perform their functions in a correct and orderly manner.
-Constitution on the Sacred Liturgy, Sacrosanctum Concilium # 28-30