In liturgical celebrations each one, minister or layperson, who has an office to perform, should do all of, but only, those parts which pertain to that office by the nature of the rite and the principles of liturgy.
Servers, readers, commentators, and members of the choir also exercise a genuine liturgical function. They ought to discharge their office, therefore, with the sincere devotion and decorum demanded by so exalted a ministry and rightly expected of them by God’s people.
Constitution on the Sacred Liturgy, nos. 28-29
Ministries at Mass
The office of priesthood is essentially linked to the Eucharist. Therefore, the service of the priest is unique in the Eucharistic Assembly. His participation is pivotal. By virtue of his office, he convenes the Christian faithful for the celebration of Mass. There he presides at the Liturgy of the Word and the Liturgy of the Eucharist, and sends the Christian faithful forth to love and serve the Lord.
The description of the offices and roles of individuals who assist the priest in their participation at Mass is explained in detail in the General Instruction. (GIRM, #91-111) of the Roman Missal.
Ordinarily the deacon, the lector, and the acolyte are the persons who assist the priest by virtue of their office and ministry. (GIRM, #94, #98)
In extraordinary circumstances, which are common today, assistants, namely Readers in the absence of instituted Lectors, and Eucharistic Ministers in the absence of instituted Acolytes, assist the priest, according to the General Instruction. (GIRM, #100, #101)