The two prepositions, OPPOSITE and ACROSS FROM, can be often used interchangeably. They both mean on the other side of sth. In the following examples either can be used:
He was sitting opposite his wife at the table.
He was sitting across from his wife at the table.
The school is opposite the church.
The school is across from the church.
I just bought a house opposite the park.
I just bought a house across from the park.
There are cases, however, when only the preposition OPPOSITE may be used. For example, if you want to say that an actor played with another actor in a movie or play, you can use this preposition, but not ACROSS FROM. Here OPPOSITE is interchangeable with the preposition WITH:
Billy G. stars/plays opposite Sarah T. in the new comedy.
Billy G. stars/plays with Sarah T. in the new comedy.
And one more thing, which is pretty much obvious, I think. OPPOSITE is not always a preposition. It may be an adjective, adverb or noun as well. In all these cases it’s not interchangeable with ACROSS FROM:
adjective: The opposite bank of the river is very steep.
adverb: I sat down and she sat opposite.
noun: He said it would be cheap, but it was quite the opposite.
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