This section gives a good perspective on external and internal participation. The council, while acknowledging the importance of participation in liturgy, also recognizes the aspect of the spiritual life which is internal:
The spiritual life, however, is not limited solely to participation in the liturgy. The Christian is indeed called to pray with (other believers), but … must also enter into (her or) his chamber to pray to the Father, in secret (Cf. Matt. 6:6.); yet more, according to the teaching of the Apostle, (she or) he should pray without ceasing (Cf . 1 Thess. 5:17.). We learn from the same Apostle that we must always bear about in our body the dying of Jesus, so that the life also of Jesus may be made manifest in our bodily frame (Cf . 2 Cor. 4:10-11.). This is why we ask the Lord in the sacrifice of the Mass that, “receiving the offering of the spiritual victim,” he may fashion us for himself “as an eternal gift” (Secret for Monday of Pentecost Week.).
From the context, one might say that for the Christian, interior prayer should be constant, as Saint Paul advises. Interior participation at church is just an extension of what the believer should be practicing everywhere, all the time.