When a background character is described in such a way that you know who it is without their name being mentioned.
I was sure that I had guessed correctly that Dr Evan Wilson, in "Uhura's Song", was based on Bjo Trimble, "the woman who saved Star Trek". The physical description (short stature, freckles) and cheeky demeanour reminded me so much of Bjo.
I wrote to Janet Kagan to ask her and she said that she had never actually met Bjo, but that Dr Wilson was based on her own mother.