Honestly, it depends who I'm talking to and what we're talking about at the time.
For instance, my mother and I just watched Earthlings. After, I launched into a teary "I HATE PEOPLE WHY ARE THEY SUCH HORRIFIC, SELFISH, SOULLESS *******S?!!" talk/rant/sobbing idiotic rambling.
I would not have launched into that in a casual conversation with someone.
For the record, I've had many genial conversations about my veganism with my mother as well, both before and after we watched Earthlings. And I JUST got off the phone with her, and she said she was going to email me (so I'd "have it") a talk about the things she feels she can change in her life to live a more veg*n, compassionate life. She's a smart woman, and it's been my experience that if you present information about animals to smart people without attacking them, you stand a good chance of getting a positive response. The key, I've found, is to present information without being harsh, demanding, or judgmental.
In terms of those quick-fire responses needed for casual conversation about, "Wow, why are you eating THAT?" I usually just shrug and say, "I don't need to eat animal products to survive or to thrive. Even the ADA and AHA say so. That means eating meat or milk or eggs is a choice. And I personally don't feel comfortable choosing to kill or harm an animal if I don't have to. That just feels wrong to me."
It's simple, it's straightforward, it's the truth. It's a remark about my own personal decision. It's a lot of "I, I, I" not "you, you, you." It's not even about the treatment of animals, which some people want to ignore. It's about my very basic ethical philosophy of not killing or harming a living creature for no reason. And since I don't NEED to, then it truly is for no reason.