Liam Hemsworth and Rebel Wilson Swap Spit on 'Isn't it Romantic' Set

Liam Hemsworth and Rebel Wilson's makeout sesh got sloppy ... even in a tux and gown.

Liam and Rebel were on the set of "Isn't it Romantic" in NYC ... making out for a scene under drenching rain and a green screen as their backdrop. It's unclear what the green screen was needed for ... but the flick's about a woman who mysteriously gets sucked into a romantic comedy. So, use your imagination.

Rain, rain go away? Not if you're in a movie.