Mac tries to look cool and dresses as Viggo Mortenstein from the Lord of the Rings movies.
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Charlie attempts to dress like a vampire when he puts on a Phantom of the Opera costume on Always Sunny.
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Artemis and Frank have a little lovers quarrel in costume during the Always Sunny Halloween-themed episode.
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A picture from the Halloween episode of It's Always Sunny episode, "Who Got Dee Pregnant?" Dee is pregnant and the guys try to figure out who the father is.
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It's Always Sunny in Philadelphia Season 6 Episode 7 Quotes

Mac: I browned out that evening.
Frank: Browned out? What's browned out?
Mac: It's when you drink so much that everything goes brown. It's not as sever as a black out because I remember bits and pieces. I call it browning out.

Dennis: It's fatness, plain and simple. It's a person becoming fat before your very eyes.
Charlie: And I don't even know how to make the bird jokes anymore. They no longer apply.
Dee: I'm not fat, I'm pregnant!
Mac: I feel like you say that all the time now.
Frank: You better do yourself a favor and flush it out.
Charlie: Wait a second, you definitely said that before.
Mac: Since we're all saying things we say all the time. I'll just reiterate. Dee, we don't care about you, or your body, or that baby bird.