When believers get together, what should be the topic of conversation? (Jesus, as much as possible!) Is it real fellowship when they just talk sports or the weather or news events? [N.B. I am not referring to fellowship with the Father and the Son, but fellow brethren.] Genuine fellowship (from the Greek word koinonia which means having something in common: Christians have Christ, a mission, ministries, purposes, the Bible, etc. in common) which is two fellows in the same ship, as it were facetiously, is when honesty, authenticity, and real sharing takes place in the name of the Lord ("For when two or three are gathered together in my name, there I am in the midst of them," reads Matthew 18:20). Sometimes it involves confession or empathizing and/or sympathizing. Consoling each other as an example is fellowship (we are comforted so that we can comfort others--2 Cor. 1:20), but that can even take place even among nonbelievers in a technical sense.
There is fellowship, and then there is fellowship. What we ought to have in common is not the weather or our team, but our Lord. Is the Spirit of the Lord present is all that matters, and if you have discernment in the Spirit you should be able to tell if the Holy Spirit shows up or is a "no-show" to your fellowship. Sometimes all it takes is the mention of His name because He is not far from each of us and is in us already.
But we stifle the Spirit and hamper His free expression and quench His presence by our conversation--we need to be sensitive to what is pleasing in His sight (cf. Psa. 19:14). The Spirit does not lie, exaggerate, jest, or tease, etc., but is full of love and that is the key to His presence in essence--Christians love the brethren, and His Spirit bears witness with our Spirit that we are the Sons of God. The believers will know if they had fellowship, but an outsider would not be able to judge or discern, for he is unaware of spiritual things and blind spiritually.
There is no limit as to how far we can go into the realm of the Spirit if we but open the door and be willing to confess Him before man and be open and free in our spirit ("Where the Spirit of the Lord is, there is liberty,' says 2 Cor. 3:17). When we are having fellowship, we are equal in God's eyes and there is neither Greek nor barbarian, slave nor free, male nor female, etc. Rank has its privileges, but not in Christ--we kiss our social position goodbye. There is often a so-called esprit de corps of the Holy Spirit, you might say. The possibilities are limitless and sometimes even ecstasy and euphoria are possible, as well as the other extreme end of the spectrum which is grief--some people don't feel they are really friends till they have cried together, not laughed together.
In summation, we must differentiate between surface-level chitchat and real genuine fellowship, which is not a given, even among believers--they may be out of fellowship with God. We must put Christ into it or be inspired or led by Him in our conversation. We share our walk and can relate to each other as to how we are doing, which is also key; but NB: There is no fellowship (in my interpretation of Scripture) with an unbeliever: "For what does light have in common with darkness? What does a believer have in common with an unbeliever?" (2 Cor. 6:14-15). Soli Deo Gloria!