And when they had prayed, the place was shaken where they were assembled together; and they were all filled with the Holy Ghost, and they spake the word of God with boldness.
Note 5 at Ac 4:31: The disciples had already been filled with the Holy Ghost (Ac 2:4), so what was happening here? The Holy Spirit resides in our spirits, and once He comes, He doesn't leave (Joh 14:16). The initial filling when the Holy Ghost first comes is what the disciples experienced on the Day of Pentecost.
However, the Holy Spirit's control and influence over our souls and bodies fluctuates proportionately to how well our minds are renewed to His will (Ro 12:2). In that sense, we can be more full of the Holy Ghost at one particular time than another, although the presence and power of the Holy Spirit does not come and go in our spirits. Therefore, even after we receive the baptism of the Holy Ghost, our souls and bodies will sometimes stray from the leadership of the Holy Spirit, and we will need, once again, to be filled with the Holy Spirit.