The interlude can actually be many different moments in the ceremony, a loose definition could be any time there are no words being spoken for a minute or more. For most ceremonies, the interlude is the unity candle, memorial candle, or rose presentation. These are moments of quiet reflection, and the music you choose should encourage this mood.
Catholic Wedding
Offertory and/or Communion
Presentation of the Flower to the Virgin Mary
Unity (Candle, Sand etc)
all of the songs can be interchangable
The Prayer, Josh Groban 