Basic Trope: A loveless couple cannot split up for whatever reason, so they agree to keep up appearances of a happy marriage.
Straight: Alice and Bob no longer love each other, but pretend to the outside world to be happily married, for social reasons - while getting on with their separate lives.
Exaggerated: Alice and Bob are married and hate each other's guts. They pretend to be Sickening Sweethearts to others, while secretly plotting one another's murder.
Downplayed: Alice and Bob have some arguments in their marriage, which they don't share with anyone else.
Bob or Alice has wealthy and conservative relatives from whom they expect to inherit, and who does not approve of divorce.
Bob and Alice live in a society where divorcees are social pariahs.
Bob and Alice only married to look good as a couple.
Bob and Alice are happily married, but stage arguments and so forth to pretend to the outside world that their marriage is failing.
Alice and Bob's private life at first seems to be loveless, but then it turns out to be Slap-Slap-Kiss.
The charade of a happy marriage breaks down.
It isn't really love, it is unbalanced violent lust, and in the meantime they both hate each other's guts.
Alice and Bob hush up the scene and pretend it was just a domestic argument.
Zig Zagged: ???
Alice and Bob make no attempt to show others that they love each other.
Alice and Bob are happily married.
Enforced: The author wants to show that the world Alice and Bob live in is a Crapsaccharine World that emphasizes appearances over substance. Having marriage that seems happy is one of the many ways he can prove that.
Lampshaded: "I hate you, Bob. I hate you and I pretended to love you for the last ten years."
Exploited: Alice finally kills Bob, knowing that nobody will believe she did it. They were such a happy couple, after all...
As soon as Alice and Bob realize they don't love each other, they divorce.
They never married in the first place because they realized they weren't right for each other.
Conversed: "Remember that show where they looked like happiest couple on Earth on the outside, but plotted each other's murder when alone?"
Implied: The couple is only seen during the parties, when they look as if they were Happily Married. However, sometimes camera shows them when other characters can't see them and they look at each other with murder in their eyes. It's never explained.