Tips for interrogation: if you can get Death Squad uniforms, use them prodigiously. You can see
here (if you can read fairly explicit c++) that they give a bonus of 8 points to all interrogation, and 6 points for interrogation assaults (except, strangely, this value is never actually used - in interrogations at least).
If you are using drugs however, then a mask or bondage gear is definitely the way to go. You get a 4 point bonus to your drug use boost (the same as with a clown, elephant or donkey suit) with either 4 or 6 points added to your base interrogation ability. The 6 point masks are: Cheney, Guy Fawkes, Jason, Satan, Jesus and Bin Laden; the bondage gear is also 6 points. If you can get those masks they beat or equal all other costumes (excepting the Death Squad uniform) with or without drugs involved. If you can't find those masks or the bondage gear, use a lab coat, expensive suit/dress, black dress, police uniform, police armour, army armour, military gear, or a prison guard uniform. But, since you can buy bondage gear from the shops for $350, it's the best choice, really.
There is a lot of code for the interrogation section, which I may wade through at some other time. Essentially, the lead interrogator is chosen from all interrogators in the safehouse; they always have the highest preliminary interrogation score but if there is a tie it is chosen randomly. So if you have two interrogators with otherwise equal preliminary score you can swap their armour to make one a certainty. Rapport doesn't factor into who the lead is, so use one lead for any beatings and a different lead when you attempt to convert, otherwise you might waste a day with a lead who the subject hates.
TL;DR: use bondage gear, mass interrogators (reduces risk of escape, among other things), and switch to a new person for conversion if you beat them first.