In an episode of Star Trek: Deep Space Nine, Odo (Rene Auberjonois) is in love with Major Kira (Nana Visitor) and does not know how to court her. He seeks advice from a character in the holodeck who is modeled on a Rat-Pack-type Vegas singer from the mid-20th Century. Over the course of the episode this singer turns in beautiful renditions of a series of songs associated with Frank Sinatra. And a hologram that resembles Major Kira (the hologram also played by Nana Visitor) sings a torch song that I cannot recall.
The male singer was played by Jack Darren as Vic Fontaine
Were these song recorded for the episode, or did they just cast a professional singer and use his album tracks for the show? Did Nana Visitor actually sing the song that her character sings, or (as I think they sometimes do in Disney films) did they use a professional singer for just the songs, with another actor appearing and performing the spoken parts?