Interrogation is instant (at least that used to be the case, but I don´t think that has chanced with 5.5X)
You get few espionage points for crew memebers, officers, on the other hand, mmmmh yummy! The higher the rank, the more intel you get
In addition to the Espionage Points generated by teams, you can also gain Espionage points by interogating captured prisoners. For every ten crew you rescue from life pods you get one Espionage Point. Every captured officer will generate a number of Espionage Points equal to his rank cubed. So if the lowest rank is lieutenant commander, he will only be worth 1 Espionage Point (1x1x1). A commander would be worth 8 Espionage points (2x2x2) while a captain would be worth 27 Espionage points and a Commodore 64 (no computer pun intended ) Espionage points, etc. With this rule, sending a Vice Admiral (216 EP) into harms way because he has a high crew training attribute may not be a great idea. It will also become very worthwhile to rescue your crews to avoid them being captured by the enemy.
Fore every 100 espionage points, you get a chance to learn something (this is the same rule as for espionage teams, which is where I pulled this one from)