Improving indoor portrait lighting beyond a basic two-light setup

[Stephen_B]

I'm a hobbyist photographer trying to improve my indoor portrait work, and I've hit a wall with my basic two-light setup that's leaving my images looking flat and uninteresting. I understand the concepts of key and fill lights, but I'm struggling with more advanced portrait lighting techniques like creating effective Rembrandt lighting or using a hair light without causing lens flare or overpowering the main subject. For photographers who specialize in studio portraits, what are the most common mistakes beginners make when positioning modifiers like softboxes or reflectors, and how do you methodically adjust your lights to sculpt the face and add depth without overcomplicating the setup? I'm working with a modest home studio space.