Q: What's the proper response to a person who casually mentions that she's in a program like Alcoholics Anonymous? This has happened to me twice.

A: A good reply is, "How is that going for you?" The A.A. member is revealing that she suffers from a debilitating illness and is undergoing treatment. That's no different from her announcing that she's in chemotherapy, and usually stems from the same reasons: She's in the fight of her life, and she wants to talk about it. Let her.
— Jack Marshall, president, ProEthics 

I find that anytime I've talked to someone about her experiences in A.A., it's been healing for both of us. There's no "appropriate" response. Simply accept the information as the gift of intimacy that it is. If you're surprised, say it: "Really? I didn't know you're in A.A. When did you join?" If you're intrigued, ask away: "What was the moment you knew you needed help?" That's not prying; that's learning.
— Faith Salie, host, public radio's Fair Game podcast

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