Candid portraits can be easy or tough. Ambient weather conditions, surrounding people; friends-family-distractions, etc.
If possible I hope to develop a micro relationship (rapport) as quickly as possible to get a sense of the subjects "inner workings". Do they have a sense of humor, are they cynical, are they eccentric, etc? My personal goal is to be personally satisfied with the technical outcome but also capture the essence of the person. Nothing greater than someone who just had their photo taken to say "wow, that is the best photo someone has ever taken of me"....And, they would never realize that 40% or less of that actually comes from the camera.
Last night I had a quick little job for a local politician - Her hair looked atrocious, the wind was gusting 30mph+ and there were tons of kids to corral around her. The end of the day I got shots because I could connect with her and get her to relax, breath and take direction. Which can be impossible. I never wanted to have a studio for portraiture stuff mostly because most people are in such denial of who they are and what they actually look like and are stuck blaming everyone and everything for their lot in life. Thus, blaming the photographer for being overweight or looking old etc.
But, in a candid situation, I love taking spontaneous portrait shots.