Currently available on DVD: "Cyrus"

John C. Reilly, Marisa Tomei and Jonah Hill give such wonderfully satisfying, full-blooded performances in Cyrus that it seems almost churlish to wish this creepy little Oedipal comedy were a little more well-thought-out, and handled its wilder shifts in tone with more finesse.

Cyrus is the much-touted mainstream debut of directors Mark and Jay Duplass (The Puffy Chair) — pioneers of the minimalist, DIY “mumblecore” movement, who are now improbably working with another pair of brothers, action experts Ridley and Tony Scott (The A-Team) as executive producers.

Reilly and Hill are veterans of the Judd Apatow school of take-no-prisoners comedy, and it’s interesting to imagine how Cyrus would have turned out in the hands of Apatow or one of his disciples.

The film initially heads off in this raunch-filled direction, as Catherine Keener (another Apatow alumna) surprises the slovenly, middle-aged Reilly — still her best friend seven years after the end of their marriage — during an intimate moment after she lets herself into his home.

Keener, who is planning to soon marry her long-suffering boyfriend (Matt Walsh), virtually orders her perpetually depressed ex to attend a party, where he meets a gorgeous woman (Tomei) while urinating into a bush.

Somehow his penchant for oversharing charms Tomei, and the actress makes her attraction to this shlubby guy credible with her warm performance, despite little help from an underwritten script.

Things get complicated when Reilly follows her home after a blissful night together. There he discovers she’s living with an emotionally withdrawn 21-year-old son named Cyrus (Hill) who’s way, way too attached to his mother.

A New Age musician, Cyrus doesn’t take the arrival of this potential stepdad well at all — Reilly’s shoes disappear after he spends his first night at their home, and things get weirder and weirder from there.

Hill, with his hair trimmed short, projects a menace that seems to signal that this is going to be a horror movie.

Then the Duplasses head back in the direction of uninhibited comedy as the son catches the interloper shagging his mom, and the two guys basically declare war on each other.

Some of these scenes are hilarious indeed. But the filmmakers don’t know how — or don’t want to know how — to segue into the more serious sequences, which are filled with too much touchy-feely talk.

Really, it wouldn’t have hurt to give these characters a few lines of expository dialogue to explain their odder choices, or to smooth out a few more of the rough edges.

Flaws aside, Cyrus is still one of the very few mainstream movies out there with any ambition, and you have to respect that.