"According to Catholicism, how does God manifest and interact with Christians?"
I agree with the statement made above, that this is a broad (and somewhat vague) question. I will try to answer it in a general way.
In general, God interacts with members of His Church, through the Mass, the Sacraments (especially the Holy Eucharist, and Confession), through Eucharistic adoration, and through the teaching authority of His Church (The Magisterium). See the gospel story of the Good Shepherd (John 10). Also, John 10:16, and Hebrews 6:4-6 may apply here.
I don't know if the Catholic Church has any standardized, printable list of guidelines that show us how to interact with other Christians (although certain parts of a standard guide to confession might qualify for that).
Many times in Catholic writings I have come across a phrase something like "Whatever we do, do all in things in charity". I think that this is probably the best general answer to a very general question.
This general idea of love (charity) is reflected by St. Paul in the gospels which do give us a general rule to follow.
1 Corinthians 13:1-8, 13 NIV
1 If I speak in the tongues of men or of angels, but do not have love, I am only a resounding gong or a clanging cymbal. 2 If I have the gift of prophecy and can fathom all mysteries and all knowledge, and if I have a faith that can move mountains, but do not have love, I am nothing. 3 If I give all I possess to the poor and give over my body to hardship that I may boast, but do not have love, I gain nothing.
4 Love is patient, love is kind. It does not envy, it does not boast, it is not proud. 5 It does not dishonor others, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. 6 Love does not delight in evil but rejoices with the truth. 7 It always protects, always trusts, always hopes, always perseveres.
8 Love never fails. But where there are prophecies, they will cease; where there are tongues, they will be stilled; where there is knowledge, it will pass away.
13 And now these three remain: faith, hope and love. But the greatest of these is love.
One last late thought. An additional thought to what I initially wrote. In the past, Christ has actively interacted with many of the saints, through inspiration, and/or actual in person visits. Sometimes He sends his Mother, the Virgin Mary. The following link will give you more info. ncregister.com/blog/saints-who-saw-heaven-and-hell –
2 days ago