LOS ANGELES (Reuters) - The script never got in the way of a good fight as comic actors Will Ferrell and John C. Reilly improvised their way through family warfare in the movie "Step Brothers," which opens on Friday.

In making the film, director Adam McKay served as ringmaster for ad-libbed scenes of sibling rivalry between Ferrell and Reilly, who play immature adults each living with a parent and forced to coexist when one's mother and the other's father get married.

McKay worked with Ferrell, 41, and Reilly, 43, on the 2006 hit comedy "Talladega Nights: The Ballad of Ricky Bobby" -- another heavily improvised film -- and teaming up again allowed the trio to reprise their impromptu comic style.

"We had so much fun working on 'Talladega Nights,' the three of us, and we really made a pact, a blood pact, to try to work on something else together," said Ferrell, a veteran of the NBC sketch comedy show "Saturday Night Live" who also starred in the movies "Semi-Pro" and "Stranger Than Fiction."

Ferrell said McKay put improvisation ahead of the script, which the two co-wrote.

While other directors throw in the occasional ad-libbed take but stick to the script, "We invert it," Ferrell said, "and start exploring things that ..."

"Really shouldn't be explored," Reilly interjected.

McKay worked as a writer on "Saturday Night Live" when Ferrell was on that show. The two last year launched the website Funny Or Die, which posts videos from professional and amateur comedians. Cable TV channel HBO has invested in the site and plans to air content from it.

In the Sony Pictures movie "Step Brothers," Ferrell and Reilly co-star as live-at-home adults -- middle-aged guys who behave like big kids, with pastimes that include hanging out in a tree house and firing slingshots.

Thrown together, the two stepbrothers start off on a confrontational footing, taking swings at each other with baseball bats and golf clubs. After realizing they are more alike than different, they become friends and create chaos for their parents, then team up to save their parents' marriage when it sours.

Playing the two parents are actress Mary Steenburgen, 55, an Oscar winner for "Melvin and Howard" who starred in "Elf" with Ferrell, and Richard Jenkins, 61, who starred in the HBO drama "Six Feet Under."

Steenburgen honed her improv skills on the HBO comedy "Curb Your Enthusiasm." But the actress said she had to work hard to keep up with Ferrell and Reilly, whose improvisation prowess she compared with the boxing talents of Muhammad Ali.

"I was nervous at the very beginning, because it felt like in terms of improvisation it was like getting in the ring with two Alis or something, because they're pretty amazing at this," Steenburgen said.

(Reporting by Alex Dobuzinskis: Editing by Steve Gorman)