It doesn’t matter if I’m by myself and someone on the radio says “inconceivable,” or if I’m entertaining the Queen of England and she says the same word. I must, as surely as I will exhale and have a pulse, say, with my tongue between my teeth, “INconTHEEVable.”
Every. Time.
I also never say I’ll be back. I will always be Bach.
Anytime anyone uses a sentence with the word “surely” in it, I will - nay, I must! - say something that includes the phrase, “And don’t call me Shirley.”
Whenever someone mentions having two of something, so it should be enough (eg “I have two bottles of wine, that should be enough for the dinner, right?”) my reply, delivered in a slurred Southern Val Kilmer is always “Ah have two bottles of waahn…one for eacha ya.”
Yup. It’s always, “Ah’ll be bach.”
Also, whenever I get several dimes in change, or someone mentions a quantity of dimes, I always have to say, “Send somebody back to town to get a whole shitload o’ dimes!” (Slim Pickens in Blazing Saddles)
Anytime someone uses the number 37 the correct response is “In a row?”
With truly dedicated fans, this can be done with any number spoken on its own. This results in much laughter.
“I need those numbers for the regulators at the SEC by tomorrow.”
“REGULAAATOORS!! MOUNT UP!!”
“you killed my xxxx” is always followed by “prepare to die”
I get one demerit for leaving that out 
“How do you know she’s a witch” will ALWAYS get me to say “she turned me into a newt!”
What do you mean, “you people?”
This, plus repeating whatever somebody says in a deadpan voice whenever someone ends a sentence with altogether.
‘Inconceivable’ is one of them for me too, though I would automatically, without thinking, say something like ‘I do not think that word means what you think it means.’
It’s hard to discern my regular speech from my geeky speech though, TBH. My friends and acquaintances quote each other quite a lot.
They aren’t actually consecutive lines, but if someone exclaims “Burma!” I find myself compelled to say “Intercourse the penguin!”
You may wonder what set of circumstances would cause someone to exclaim “Burma!” (other than if they panic, of course), but this has actually happened when it was the answer to a game show question on TV.
That’s where I thought the OP was going with that, too.
And it doesn’t come up as often, but an exclamation of “Werewolf!” must be followed by “There wolf”.
Everytime I hear someone say “Damn it!” I’ll be hard pressed not to follow up with “Janet! I love yooooou!” Sadly most of the times I do actually do it, I get blank stares 
On occasion someone telling me to bear left will get a response of “'Frog right!”, usually to their total incomprehension. Kids these days, no respect for the classics.
It’s hard to get through a work day without a Glengarry Glenn Ross quote.
Coworker: “What are you going to do about it?”
Me: “Asshole.”
::Coworker drinking coffee::
Me: “Put. Down. That. Coffee. Coffee’s for closers.”
I’m like Pavlov’s dream experiment for this kind of stuff. conditioned replies all over the place.
One I haven’t seen anyone mention yet, though, is one of my most consistent :
“Who ya gonna call?”
I actually had the occasion to hear someone say “POH-TAY-TOES” when her husband didn’t hear what they were planning on eating that night. Unfortunately she didn’t understand when I replied “boil em, mash em, stick em in a stew” after she finished her call. She had just said that spontaneously without meaning to reference LOTR.
If anyone says, “To make a long story short.”
I say, “Too late.”
Clue.