4.1.6 Preaching and Reading in Various Ecumenical Situations
1. Catholic priests, deacons and laity are not to preach at Eucharistic or similar celebrations of non-Catholic Churches and ecclesial communities. There is not to be pulpit sharing at these types of celebration.
a. These include Eucharistic celebrations under the various titles of Divine Liturgy, Lord’s Supper, Holy Communion, etc., as well as the principal liturgical service of the Word held by an ecclesial community to observe the Lord’s Day. (It would include other occasions, such as baptism, wedding or funeral when included within them.)
b. Moreover, care is to be taken to avoid any appearance of the concelebration of the Eucharist with priests or ministers of Churches or ecclesial communities that are not in full communion with the Catholic Church. 
2. Catholics, if invited by their hosts, may read a lesson or preach in other non liturgical celebrations that take place in other Churches and ecclesial communities. 
a. These include, illustratively, the occasions of baptism, wedding or funerals, when celebrated apart from the celebration of Eucharist or similar communion service or the principal liturgical service of the Word held by an ecclesial community to observe the Lord’s Day.
b. Caution is to be taken to avoid conveying a false sign of unity.
3. Priests are authorized to permit a member of a non-Catholic Eastern Church, or a baptized member of an ecclesial community, to serve as a reader or proclaim the general intercessions when they celebrate the Eucharist or other act of divine worship, such as a baptism, a wedding, a rite of Christian funeral, or an order of blessing. 
a. Permission for a non-Catholic Christian to read at Mass is to be given only for an exceptional circumstance when a just cause exists and the reader is suitable. This is not to be done on a habitual basis.
b. Deacons may extend such permission for a liturgy which they celebrate unless the pastor or rector of the church or oratory expressly prohibits it.
4. Catholics may read lessons at a sacramental liturgical celebration in a non-Catholic Eastern Church if invited to do so.  They may read the Scriptures during non Eucharistic celebrations of ecclesial communities, if invited by their hosts. 
5. “In the Catholic Eucharistic Liturgy, the homily which forms part of the liturgy itself is reserved to the [Catholic] priest or deacon, since it is the presentation of the mysteries of faith and the norms of Christian living in accordance with Catholic teaching and tradition.”  No one may provide otherwise. In the exercise of preaching, all are to observe the norms issued by the diocesan bishop (c. 772 §1). (See norms at #4.2 Preaching of the Word of God.)
In ecumenical non-sacramental ceremonies, such as vespers / evensong, Lenten or Advent gatherings, services for the week of Christian unity, etc., shared reflection and preaching may be a means by which the longed for unity is envisioned. There is no prohibition against a Catholic preaching or reading Scripture (even a gospel) in these ecumenical situations or in non-Eucharistic celebrations of other Churches and ecclesial communities.
 C. 908.
 Ecumenical Directory, n. 118 regarding non-sacramental liturgical worship.
 There are various bases for the authorization that this diocesan norm gives: some are given by the bishop as required by the directory (and which are given by this norm) while others are given by the directory itself. See Ecumenical Directory, n. 133, 126, 135 and 118.
 Ecumenical Directory, n. 126.
 Ecumenical Directory, n. 135, referencing n. 118. This appears in the section “Sharing Sacramental Life with Christians of Other Churches and Ecclesial Communities.” “Other Churches” would be taken in distinction to the earlier section regarding the “various Eastern Churches.”
 Ecumenical Directory, n. 134, and see c. 767 §1; PCAI May 1987; Collaboration, art. 3; RS, nn. 65-66; USCCB c. 766 complementary norm.