Handbook of ROCOR Liturgical Practice:

Is it permissable to serve from memory, if we have portions of the Divine Services memorised?

While it is good practice to commit the prayers of the Divine Services to memory, so that they flow from the heart as familiar words that have, to a pious degree, been made one’s own, we never actually serve without having the Service Book before us. This means that we always have the appropriate Service Book open, in front of us, as we read every prayer, litany or exclamation — even if it is there chiefly for ‘reference’ to words that we know very well.

These two practices — committing to memory and reading from the books — are not mutually exclusive. It is particularly helpful to have exclamations memorised, so that when concelebrating with a Bishop or with other clerics, the sudden ‘assignment’ of a particular exclamation to us by the chief celebrant does not have us shuffling to find words that should be familiar and already on our lips. However, relying on memory to the exclusion of having the Service Book open before us, is not a part of our tradition.

We should further add that it has always been an essential part of Russian Orthodox liturgical practice, that all concelebrant clergy are praying togetherduring the Liturgy: it is not simply the chief celebrant who reads the prayers while the other clergy stand in the Altar. This means that for all the mystical prayers, etc., all Priests should be reading the prayers from their Service Books, just as the chief celebrant is doing.