Beth's Audience Surrogate moment after Holly breaks her leg and the full gruesome mess is revealed:
Oh, that's fucking disgusting!
And immediately after, when Liz mentions she needs to push the bone back in, the deadpan reaction is darkly humorous.
I... hate you.
A Behind The Scenes example (discussed on the DVD's special features) came as the result of some Enforced Method Acting. Throughout filming, the Crawlers' appearance was kept secret from the actresses, so that their reactions would be more genuine to the first appearance. Unfortunately, this went horribly right as the women scattered off the set entirely.