I scanned a few sketches I did from photos of familiar subjects--kids I worked with for at least a year (and my cat), so I knew their personalities. They illustrate how helpful it is to have some experience with your portrait subjects. I tend not to notice the common drawing-from-a-photo issues (the tell-tale "stiffness" for instance) if I actually know the person. So I definitely advocate getting to know someone a little before doing their portrait (or taking the artist out for coffee if you're having your portrait done.) Snapping your own photos is not only helpful for finding the perfect composition and lighting that you're after, it's also a good excuse to spend some time interacting.
I was commissioned to do a portrait of a pair of kids one time. The mom wanted it to be a surprise for the dad, so she didn't want me to come over to meet the kiddos. She gave me 4 or 5 photos of each kid, including some taken outside in natural light (because I had specifically asked for that.) The outdoor shots were taken on a blazingly sunny day, so there were harsh shadows across their faces. Of course, Mom's favorite shots were ones that were taken indoors with a flash.
I never met the kids in person--never watched them run around in the yard or heard them explain their favorite Pokemon character... The portrait came out...OK. The foliage was the best part. The client was happy, but I couldn't bring myself to show it to anyone else. So, I'm never doing it that way again if I can help it. Insist on face-to-face meetings--that's just good advice for anything, if you think about it.
All future portraits that I do on commission will be totally awesome! There will just be some fine print in my order form under Expenses (if I don't know you): coffee, mini-golf outing, afternoon at the park...or something.