It's easy to forget since we've seen him lately as Dewey Cox
There relationship is a whirlwind, and the energy of newfound love is in the air. Molly seems to be holding back something, though. John can't quite put his finger on what it could be that's standing in their way. Then he discovers it: Molly has a son, Cyrus(Jonah Hill). Not just any son. He gives whole new meaning to the term "momma's boy". At 21 years old, Cyrus is creepy as hell and more than a little overprotective of Molly. It's a totally symbiotic relationship. He depends on her for everything, while she encourages his dependency at every turn. John's insertion into their life screws up their dynamic, which Cyrus perceives as a threat.
What follows could've devolved into Step Brothers-like open warfare, but instead the shenanigans are much more subtle. It starts with something subtle, like stealing John's shoes. Then it slowly snowballs into emotional manipulation that threatens to ruin John and Molly's happiness. After so many years of despair, John isn't about to stand by and watch his chance at love be trampled by a spoiled overgrown kid.
This has been an interesting year for Jonah Hill. Usually the comedic focal point of every film he, with both this and Get Him to the Greek he's taking on a diferent path. In Greek he plays a distant second fiddle to Russel Brand's crazy rocker, Aldous Snow. In Cyrus, we get our first real glimpse at Hill playing a character with many emotional layers. Cyrus could've easily been a cartoonish "bad seed", but Hill shows surprising depth by making him a character we actually feel a little sorry for. He's not evil, or doing what he's doing out of hatred, but because he sees everything he's ever known falling apart. It's easily the best Hill has ever been. You expect multi-faceted performances like that out of Reilly, and he delivers again in spades. The best part is how Reilly makes tiny changes to John's demeanor. His transformation from shy coward to fiercely determined protector occurs so naturally. Has there ever been a time where Marisa Tomei wasn't absolutely beautiful? Has she ever played a character that you didn't want to wrap your arms around and protect?
If you're expecting far out hijinks, Cyrus isn't going to be your cup of tea. The Duplass's don't do slapstick. At least not yet. That doesn't mean their aren't plenty of big laughs. If you haven't seen any of their previous films, consider Cyrus the best place to start.