The most 'rote' of all movie formulas, I think we can all agree, is the Romantic Comedy. I read an article somewhere that defended this, saying that women liked the comfort of knowing the formula and didn't really want any surprises or unexpected twists and turns.
At any rate, the Acts usually break down like this:
Act 1: The two romatic characters meet, often under some sort of a false pretense. The threat of the false pretense, or other force working against the couple, is also introduced, but is not a serious threat in this act.
Act 2: The couple come together, at least momentarily, and every thing is going well. The underlying threat of the false pretense being revealed culmiantes until the end of the second act, when the couple splits.
Act 3: As soon as they are broken up, you know the third act has started. Whatever the damage was that ended the 2nd act, must now be quickly resolved- possibly through a montage of some kind.
Epilogue/Act 4 - The Wedding or equivalent. Often in slow motion, or with just light piano music in place of dialogue. Roll credits.