Norms for Ministry:
Whether clergy or laity, in liturgical celebrations, each person who has a function to carry out should do all of those parts that pertain to his or her function by the nature of the rite and the principles of the liturgy, in fact, only those parts that pertain.
Servers, lectors, commentators, and choir members also exercise a genuine, liturgical ministry. Therefore, they should carry out their role with sincere piety and decorum, demanded by so exalted a service 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 or her own measure. They must be trained to carry out their functions in a correct and orderly manner.
(from the Second Vatican Council, Constitution on the Liturgy, nos. 28-29)