Q: In the absence of a deacon, and when one or more priests are present to concelebrate, a concelebrant is supposed to take his parts of the deacon (GIRM 208). Does he read the gospel? Does he prepare the altar? Does he assist at the eucharistic prayer? Invite the sign of peace? Does it make a difference when the bishop is the presider?
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A: In this circumstance, the following would apply:
A priest receives the blessing from the bishop and then proclaims the gospel—but he does not receive the blessing if the principal celebrant is another priest.
In the absence of a deacon, the petitions in the prayer of the faithful more fittingly go to a layperson rather than to a concelebrant.
A concelebrant may prepare the altar, but a server or master of ceremonies may do so. However, a concelebrant should add water to the wine and say the appropriate prayer.
At the end of the eucharistic prayer, a concelebrant should lift the chalice.
A concelebrant should invite the sign of peace.
A concelebrant should give the dismissal.
This will surprise most priests, but it was what the GIRM expects.