"A fanatic is someone who can't change his mind and won't change the subject." - Winston Churchill.
You're at a party, or perched on the bar, or standing around a grill. There's beer. Someone starts talking about beer. And, almost irresistibly, you find the words spilling out of your mouth, despite any intention you had of keeping them in...
"Actually, I brew beer."
If that's not you, then you can go. But I can write that sentence fully aware that I'm keeping an enormous percentage of you on board for this, because as far as I can tell we nearly all do this. It's not inherently a bad thing, but it certainly has the potential. Homebrewer fanaticism might not be what it once was, but there's still plenty of us out there.
The Downside of Depth
Specialty is the enemy of gaiety. It isn't so much that people aren't or couldn't be interested in brewing (though they might not be), it's that the kinds of conversations homebrewers tend to have about beer and brewing are only interesting to other homebrewers, and even then it may be heavily dependent on the other's brewing practice, experience, method, or skill.
If I'm not a Brew-in-a-Bag guy, then there's only so much I care about where you're ordering your bags from and whether or not they eliminate the need for rice hulls in a wheat-heavy grist. If I don't use extract I'll have a hard time mustering up interest in Muntons v. Alexanders or DME v. LME. If I don't use induction...well, I do, but you get the point.
And that's assuming you're talking to another brewer. Now imagine you're not. How much do they care about anything other than the very, very basics of brewing, if that much?
If you're talking about homebrewing, there's a very good chance that you're not educating or entertaining - you're masturbating. And no one wants to see that in public.
Courtesy v. Interest
I'm sure I've said this before, but never mistake courtesy for interest. Yes, you're all talking about beer. It can be fun, and if you're at good craft beer bar or brewery there's bound to be some casual interest. This shouldn't convince you that you have a green light to go full beer geek.
If you're going to talk about brewing, maybe don't go full beer geek. If you use terms that need to be explained, you've probably already gone too far. If you mention ales v. lagers, take a breath and change the subject. If you think you might let the words SRM, sparge, or Vorlauf escape your lips, step outside and punch yourself in the genitals until you feel the sensation pass.
The danger here is that people might keep listening just to be polite. Hell, I'm married to someone who keeps asking questions about something even when she hates the topic, just because it seems polite. Don't assume that just because they're nodding and smiling that they're not screaming on the inside.
A Matter of Taste
I've been down this road. I know what it feels like to be the overcommitted, hyper-talkative alehole in the room. You realize it the next morning, or maybe later that night, and you wonder why you didn't just stick to the basics.
So that's my advice. Stick to the basics. Hell, stick to less than the basics.
I prefer to talk about beer like it is what it is - a food product. Leave the science and the mechanics aside completely. I've never, not once, had a brewing-based conversation that focused on taste and flavors that has gone awry. But I've seen (and ignored?) a lot of glazed eyeballs when I go into too much detail on the question, "how long does it take to make beer?" ["Well, only about 4-5 hours to make it, all-grain, maybe 2-3 if it's extract, but you want to do all-grain because of the control you get. It's cheaper, too. Anyway, all-grain is when you do your own starch conversions, instead of just getting the liquid or dried extract. Hey, cool - where'd you learn to tie a noose? So, as I was saying you can use a cooler or a kettle to mash in, mashing being when you hold the grain in a warm, wet bath so the enzymes can convert the starches into - sure, I'll get you a glass of water for those pills - simple and complex sugars. See, yeast can consume and convert short-chain sugars but not long-chain sugars, so there's some left over which is why we need hops to balance the sweetness of the remaining - oh, Bob? He's in the bathroom with my pocket knife, I think he needed to clean his fingernails - long-chain sugars and alcohols, because you know alcohol is sweet, too! Anyways, that's just the brew day, but it can take anywhere from a few days to several weeks to ferment and age the beer..." You get the idea.]
And when I say to talk flavors, I don't mean "round maltiness" or "flinty bitterness" or "slick diacetyl." I mean "pineapple," "banana," "coffee," and the like. Imagine you were describing cake to someone who just generally, sort-of-knows-about baked goods. You'd talk up the sweetness of icing and the softness of sponge cake, not the advantages of convection ovens over gas ovens or silicon trays v. aluminum, right?
Nothing wrong with something basic ("all alcoholic drinks are basically just sugary liquids, fermented, which makes alcohol") and a discussion of flavors ("lots of beers taste like banana or spices even without using fruit or spice"). Crack that tiny egg of knowledge, then shut up. Pretend you're cultivating an air of mystery, if it makes it easier for you. Answer questions, but don't try to gin up a long conversation. Maybe it'll happen anyway, but try to talk about other things in between brewing-related questions. And maybe you're just better at this than I am.
But I always get better results when I keep it brief. A couple of points, and then move on.
On that note, why not stop here?
Keep it simple.