There’s a scene (it might be the opening one) in the movie in which a man approaches a woman at a cafe or lunch counter type place in a major city.
He’s hitting on her, and it appears to everyone in the restaurant that they are strangers, but they’re a couple. They were role-playing.
Pretty sure this is a movie that came out within the last 15 years or so.
I thought it might have been “Unfaithful” but I found a transcript of the movie dialogue and that’s not it. I’ve tried Googling that scenario with various keywords…no luck.