What sorts of words or phrases might a Roman utter in surprise or fear? I’m making answering machine messages for a project. 
Thanks for your help!
Mehercule! or Non mehercule! it’s the Latin equivalent of oh my god!
mehercle or the myriad other expressions using his name might be uttered in response to a surprising comment, but not to actual fear. Similarly, mecastor is used in oaths, but mostly by women.
For the most part, Latin fear sounds are apparently the same as ours. They use oh! and ah! both negatively and positively, but they also have the rather amusing atat! which is often lengthened to amazing lengths. Some examples for the last mentioned: atatatae, attatatatae, atatte, atattate, attatai, attatatai. I can just imagine a Roman soldier being snuck up on by a Carthaginian and being able to do nothing but generate Morse code (atatatatatatatatatatatatatatatatatatatatat) for a minute before he can defend himself.
For the most part, Latin fear sounds are apparently the same as ours. They use oh! and ah! both negatively and positively, but they also have the rather amusing atat! which is often lengthened to amazing lengths. Some examples for the last mentioned: atatatae, attatatatae, atatte, atattate, attatai, attatatai. I can just imagine a Roman soldier being snuck up on by a Carthaginian and being able to do nothing but generate Morse code (atatatatatatatatatatatatatatatatatatatatat) for a minute before he can defend himself.
Yeah that makes sense. When one is afraid or angry they might shiver, so it would be easier to make that sound, wouldn’t it?
Salvete
How about “ei!” or “hei!” And there’s also the good old “pro di immortales!”
Here are some home pages on Latin interjections:
http://www.math.ohio-state.edu/~econrad/lang/lintj.html
http://ancienthistory.about.com/cs/latin/qt/interjections.htm
I like the “Pol!” 