I shoot 100% of my portraits with a 70-200mm IS L series lens from Canon. With that lens I can stand about 6 feet from my subject and blow whatever is behind them so out of focus that it becomes an instant backdrop. I tell people all the time "The world is my backdrop" I also use what's known in the industry a "hot light" except mine is cold. My wedding photographer Guru invented the Ice Light and it has changed the way I can contour light and shadows.
So you have your gear, you have your space and you have your subject. I will firmly say if your subject doesn't trust you, you are not going to get "the shot" Sure you may get some pics but the smile may be forced or fake looking. Getting people to let their guard down and trust you is 80% of the gig. Here's a story of a woman who started off our session by telling me right off the bat she hates getting her picture taken and isn't expecting much from the pics. I changed directions immediately and asked her about job and oddly, I happened to have a joke about her industry to which she promptly groaned, after a few minutes of just chatting I told her "let's just take a few and see what we get" I also assured her I would delete any image she didn't like right in front of her.
She believed I was going to protect her image. After about five shots, she relaxed and I got what I felt was "the shot", I showed her and if I remember correctly her response was
"hey, I don't hate that" we shot for another 10 minutes but none moved me like that one pic. Here's that image, beautiful.
I have shot hundreds of portraits and as of a few days ago, I may have technically shot one of the best of my career. Shot in a living room with a fax machine behind me and a dining room table behind him you would think I was shooting in a high end studio.
Full 120mm lens, aperture f/4.5, exposure time of 1/100th of a second, ISO of 1000.
White balanced of 5200K for the Ice Light, camera RAW, not .jpg
Here's a bigger version of a few from that session...Lance
The balance of technical and gained trust is what gets a great head shot. I will never say I've mastered either but I am loving learning every day.