A Sydney Aquarium rep tells PEOPLE's Australian sister magazine, Who, that the pair arrived around 11:30 a.m. with about six others for a tour of the Great Barrier Reef exhibit. Among the displays are clown fish (like those in Finding Nemo), and two varieties of sharks – the Blacktip reef shark and the Lemon shark.
Both stars fed the sharks by dropping the food on top of the water.
The guide who escorted the couple found them to be "very friendly and chatty and interested in learning about the fish," says the aquarium rep, who adds, "They weren't outwardly affectionate, but you could tell they were close to one another."
Aniston wore her signature basic black – a sparkling strapless Armani. Asked by a reporter about the movie's love lessons, she replied, "Oh, what have I learned about relationships? You know, you learn nothing more from this movie than I think we learn from our real relationships. Which is communicate better, be as honest as you can be, and just try to do better."
Asked whether she would consider moving to Chicago, where The Break-Up was filmed (and where she and Vaughn are rumored to be house-hunting), Aniston told Who, "I would consider moving to Chicago, yeah. It's a great city." Her favorite thing about it? "The food."
Inside the theater, she and Vaughn appeared together onstage to answer questions before the screening. On Aniston's nude scene, Vaughn said, "That was the only day I was early, I was the last one to leave, and I said (to the crew): 'Guys, I know you think we got it, but we don't want to walk away from this scene too soon.' "
"Yeah," added Aniston. "That was a fun day."
Quipped Vaughn, "I saved you all from having to see me full naked. They get to see me and all my flab, but that's not really a selling point, Jen!"
To which Aniston replied, "That depends on whatever blows your hair back!"