As women achieve more milestones toward gender parity in film (and there’s still a long way to go), steps toward equality don’t always have to be positive or uplifting images of womanhood. Sometimes, it’s more relevant when female characters can be just as raunchy, drunk, morally corrupt and beloved as their badly behaved male counterparts. “Rough Night,” from “Broad City” writers Lucia Aniello and Paul W. Downs (they co-wrote the script while Aniello directed and Downs stars), is the first R-rated Hollywood studio film directed by a woman, about women, in decades. It not only pushes the boundary of questionable behavior, it does a handstand on top of it.

In “Rough Night,” Aniello and Downs inject a lot of the strangeness of “Broad City” into the generic girls’ trip movie. The film, which starts off a bit rocky, only gets better and funnier as it builds.

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Scarlett Johansson stars as Jessica, a budding politician running for state Senate. She’s struggling in the polls against an opponent voters find “relatable,” even though he can’t stop tweeting lewd selfies. Nevertheless, she heads to Miami with her college girlfriends for her bachelorette weekend. Johansson plays the straight woman in her posse, which includes kindergarten teacher Alice (Jillian Bell), activist Frankie (Ilana Glazer), wealthy divorcee Blair (Zoë Kravitz) and Aussie flower child Pippa (Kate McKinnon).

It’s standard party girl stuff, until Alice accidentally kills the young man they’ve invited into their pad as a stripper, and they decide that instead of calling the police, they’ll dispose of the body.

Bell is a standout as the unhinged Alice, and both she and Downs (as Jessica’s fiance) turn in the movie’s funniest performances because they commit so fully, with a manic intensity, and wide-eyed determination. McKinnon is also predictably great, on her own Aussie planet. You almost wish that “Rough Night” had been given a second pass, to sharpen some jokes and smooth the transitions and edges, but the bumps in the road are easy to overlook with this excellent cast.