A: "The homily should ordinarily be given by the priest celebrant himself. He may entrust it to a concelebrating priest or occasionally, according to circumstances, to the deacon, but never to a lay person."¹ "The purpose of the homily is to explain to the faithful the Word of God proclaimed in the readings, and to apply its message to the present. Accordingly the homily is to be given by the priest or the deacon."² "The homily, therefore, during the celebration of the Holy Eucharist, must be reserved to the sacred minister, Priest, or Deacon to the exclusion of the non-ordained faithful, even if these should have responsibilities as 'pastoral assistants' or catechists in whatever type of community or group. This exclusion is not based on the preaching ability of sacred ministers nor their theological preparation, but on that function which is reserved to them in virtue of having received the Sacrament of Holy Orders. For the same reason the diocesan Bishop cannot validly dispense from the canonical norm since this is not merely a disciplinary law but one which touches upon the closely connected functions of teaching and sanctifying. For the same reason, the practice, on some occasions, of entrusting the preaching of the homily to seminarians or theology students who are not clerics is not permitted. Indeed, the homily should not be regarded as a training for some future ministry. All previous norms which may have admitted the non-ordained faithful to preaching the homily during the Holy Eucharist are to be considered abrogated by Canon 767.1."³ "According to Canon 767, preaching a homily (the sermon given after the Gospel at Mass) is exclusively reserved for bishops, priests, and deacons. The local bishop may give a lay person temporary permission to preach (Canon 766), but the preaching can never be the homily (my emphasis), which always comes after the Gospel at Mass and can only be given by someone who is ordained. Preaching in church outside of Mass is permitted for serious reasons at the discretion of the pastor and bishop (my emphasis)."⁴ "Lay persons can be admitted to preach in a church or oratory (but not during Holy Mass) (my emphasis) if it is necessary in certain circumstances or if it is useful in particular cases according to the prescriptions of the conference of bishops and with due regard for Canon 767.1."⁵ "#1 Among the forms of preaching the homily is preeminent; it is part of the liturgy itself and is reserved to a priest or to a deacon (my emphasis); in the homily the mysteries of faith and the norms of Christian living are to be expounded from the sacred text throughout the course of the liturgical year. #4 It is the duty of the pastor or the rector of a church to see to it that these prescriptions are conscientiously observed."⁶

The General Instruction of the Roman Missal does not allow for a sermon or talk by a lay person at anytime during the Mass. Priests are required to follow the format of Holy Mass as outlined in the GIRM and may never change anything in the Mass on their own.