Too much time apart? Contrasting table manners?
A certain young, beautiful co-star?
Let's focus on that last one for a bit. Rumors are circulating that Phillippe had an affair with Abbie Cornish, an Australian actress that will be sharing the screen with Ryan in the upcoming film, The Golden Age.
But did they share a bedroom, as well? On October 10, Texan Lauren Scott witnessed Phillippe and a blonde, who fit Cornish's description, kissing at sushi restaurant Kenichi.
"I was pretty shocked that he would blatantly cheat on Reese in public," Scott told Us magazine.
We're not here to judge, just to introduce Cornish to the world. She's 24. She's 5'8". She hails from Down Under, so many on our staff compare this tall actress to Nicole Kidman. Kidman's camp had no comment on the comparisons.
Cornish will soon be seen in A Good Year alongside yet another Aussie, Russell Crowe. Will she make a move on that married man, too? It's hard to say.
But we'll be here to report on the mere possibility, don't worry.