OK, so here’s a dilemma.

A few weeks ago I was in San Francisco to attend a conference. I stopped into a toy store in “Pleasant Valley,” as Laird calls the suburb where I grew up. While I was looking at Brio trains, I became aware of a guy standing next to me holding a chemistry set. I got the impression he was checking me out. For the longest time I stood there, unable to turn and face him — maybe because in my subconscious I already knew who he was. Finally I turned and, sure enough, recognized my brother.

If “brother” is the word. I’m not sure there is a word for the son of someone you live with for several years because your mother and father have married his mother and father in a technically illegal ceremony.

He was taller than I remembered, and he has lost the hexagonal glasses he had as a kid, and grown a fuzzy beard. But he still has the same blinking stare, like he is classifying you in a taxonomy of his own.

Anyway, that’s “Matt” in the book Fallen Lake. Seeing him, I felt as if I had stepped into an elevator shaft, plunging me into the basement of my recollections.

I had an impulse to run out of the store, but he said my name and in a moment he had his arms around me, and I was squeezing him back and crying a little bit.

We must have stood there talking for over an hour. He is married, which to anyone who has read Fallen Lake, may come as a surprise. He does research for a gene therapy company. I’m a doctor, so for a while we chattered shop talk. Then my phone rang. One of my patients was in cardiac arrest. Matt signaled that he had to leave and was gone before I hung up.

But since then, and especially since Fallen Lake was published, I’ve been thinking about giving him a call. I want to know if he remembers those events the way I remember them, or the way they are in the book. Most of all, I want to know how he overcame the condition he was diagnosed with as a child. But how do you even start a conversation like that? If anyone has ever been in a situation remotely like this, please give me some advice.