The dinner table scene in “Talladega Nights” alone (“Dear lord baby Jesus…”) was confirmation that Will Ferrell and John C. Reilly had a certain chemistry that needed to be explored further. Two funny men who can improvise madly, the actors were made for each other.

But “Step Brothers” is not the project they needed (even if they came up with the idea, which was then scripted by Ferrell and director Adam McKay). A simple-minded premise and lazy execution scuttle the possibilities.

Ferrell plays a 40-year-old man (not necessarily a 40-year-old virgin) who lives with his mother (Mary Steenburgen); Reilly a 40-year-old man (and I’m pretty sure a 40-year-old virgin) who lives with his father (Richard Jenkins).

The parents marry, and the “boys” are stepbrothers, sharing a bedroom and mutual dislike. This quickly turns to mutual devotion when they discover their shared affection for dinosaurs and bunk beds.

There is obviously a lot of improvising going on in this movie. In fact, you wonder if a project like this consists of a premise and a 10-page outline that says, “Will Ferrell ad-libs” on every page.

The improv is confirmed by comparing the movie with its trailers, which contain entire alternate scenes as well as lines that are nowhere to be found in the film itself. Yet for some reason “Step Brothers” doesn’t afford Ferrell and Reilly the chances to cut loose the way they did in “Talladega Nights.”

Maybe because their characters here are such adolescent morons — even more so than usual — the two actors are limited in what they can do. The expected riffs never happen, just a series of physical jokes (and extensive f-bombing — this thing is R-rated).

There are some good bits for supporting characters, including funny-creepy Adam Scott as Ferrell’s younger brother, and funny-creepy Kathryn Hahn as his lusty wife. Unfortunately, both Steenburgen and Jenkins (late of “The Visitor”) are stuck playing it straight.

“Step Brothers” is produced by Judd Apatow, whose recent run suggests an impending nervous breakdown — in 2008 alone he’s produced “Drillbit Taylor,” “Forgetting Sarah Marshall,” and the upcoming “Pineapple Express” (as well as co-scripting “You Don’t Mess with the Zohan”). Whereas Apatow could formerly be relied on for some care in his projects, things are getting sloppy now.

Apatow’s name means there must be some full-frontal male nudity, and “Step Brothers” comes through with a scene that should go down in film history. It involves Will Ferrell, a drum kit, and (I pray) a prosthetic body part. If I could wipe it from my memory, I would.