What do we mean when we say someone is 'brilliant'?
Brilliant means something shines brightly - it radiates light. As far as I can tell, the dictionary doesn't distinguish between something that reflects light (like a diamond) and a source of light (like the sun). When we think about things that shine, it seems to me that brilliant means something that we cannot look at very closely because the light it radiates make it hard to see its edges.
Think about someone you have described as "brilliant" - if at all possible, think of someone you know or someone you have known. Hold that person in your mind, and allow yourself to go back to your experience of them. Did the light that person radiated allow you to see something more clearly?
Bright lights cast shadows. What was cast into shadow by this particular person's brilliance?
Look through the light into the person. It's hard to focus on something when it shines. Maybe this is why we often add "and" to "brilliant" - "brilliant and funny" - "brilliant and troubled" - "brilliant and beautiful."
Now think of a second adjective that describes the person you are holding in your thoughts, and take another look. What do you notice different through this new adjective?