uh, these were not actually uttered by the actors, but i DID hear them in movie theaters:

at the opening of ROCKY II, from the back of the theater:
"he WINS!!!!"

Star Trek IV, when they save the whales... kirk is trying to figure out if the transporter can handle the whales and all the water:
"well, you just beamed up scotty and uhura, didn't you?"

the opening of the original halloween... jamie lee curtis keeps stabbing the boogie man, dropping her weapon, and running away:
"take the knitting needles WITH you, BITCH!"

and on the subject of horror films, some comedians' comments:

(richard prior on 'the exorcist'): exorcist wouldn't have even been a movie if it had black folks in it. regan's father would have come upstairs and said, "girl, what in the hell is that funky smell? go wash your ass and get downstairs, help your mother with dinner. and get that cross out'cha pussy..."

(eddie murphy on all 'possessed house' films): what is it with you white folks? if your house is possessed, why don't you just leave? if it was a black family, the film would have been six minutes long: "baby, i love this house. it's got lots of space, a nice yard... (deep, angry voice "GET OUT!!!!) too bad we can't STAY, though...

kenny rogerson on slasher horror: i hate those flicks. those kids are such idiots. i always root for the killer (she's hiding behind the tree, go get her!)
"oooh, an abandoned summer camp, no phones or electricity, fifteen kids were murdered there last year. EXCELLENT place for a party". and then the kid finds a head in the toilet and says, "hey, we'd better go warn the others".
fuck the others, kid. i find a head in the toilet, i'm goin' home. the others can fend for themselves.

but my mostest favorite comment was during "deathtrap", a lesser known film from the 80's starring dyan cannon, michael caine, and christopher reeve. the two guys are revealed to be lovers early in the film when, quite unexpectedly, reeve leans into caine and plants a long and fairly passionate full mouth kiss. my roommates would cry out, "ohhhhh, NO! SUPERman....."