A lot of the questions you're asking I think are best answered with your own personal experience. Having a confident vocalist, one that is comfortable with you and trusts your judgment, is the most important step. This allows you guys to experiment. Trying new things and figuring out what works and how to deal with vocalists is the experience.
Harmonies are always a plus (i mean this generally, of course). Obviously, doing a three-part harmony for every syllable across an entire album would be more monotonous than listening to James Lipton give an Autotuned speech about corn. But knowing when to and when not to add harmonies is crucial to adding excitement to the vocals. (love 3rds for mellow stuff. i try to avoid robotic "perfect" harmonies if my intentions are to have them prevalent.)
I almost always use reverb on my vocal tracks. Just to set it off the speakers a bit. The more reverb you add, the less aggressive the vocals will be. but it can be almost fatiguing when it feels like the vocalist is sitting on your head and smacking you in the face (so-to-speak).
Research and learn about singing. Become a decent vocalist (for demonstration) and really try to understand how singing works. Teach them. work with them. DO WARMUPS. and so on...