Is this the longest “Saturday Night Live” cold open ever? No, it’s “Bombshell,” a hammy new film based on the ouster of Fox News head honcho Roger Ailes for sexual harassment.

The drama about a very serious subject shoves aside insight in favor of cartoonish impersonations of Fox News personalities. Audience members giggle as they watch Jeanine Pirro, Geraldo Rivera, Sean Hannity and more yukking it up in the Manhattan newsroom, dulling the film’s potential power.

Yes, TV stars tend to be big, eccentric people, but it’s the actor’s job to go beneath the surface and find their character’s truth. That is in very short supply here.

Although Gretchen Carlson (Nicole Kidman) kicked off the case against Ailes (John Lithgow), and recorded a year of their private conversations, the movie mostly belongs to Megyn Kelly (Charlize Theron) and her struggle to speak out against her boss while also dodging Twitter attacks from Donald Trump. Theron does a fine job transforming into Kelly in a meticulously detailed turn, but she’s too unflappable, even behind closed doors.

As Carlson, Kidman speaks quietly and gives a performance that’s a dead ringer for the religious mom she played in last year’s “Boy Erased.” Her subtlety, however, is a nice break from all the schmacting on display.

And Margot Robbie plays a fictional Fox producer named Kayla, who the film’s writer Charles Randolph has said is a combination of several Ailes accusers and the result of many hours of research. She’s a conservative, evangelical superfan of the network, who’s always dreamed of working there. Her grander aspiration of being a Fox anchor leads her into Ailes’ office.

Here are some of Kayla’s so very well-researched lines: “I see myself as an influencer in the Jesus space.” And, after she mistakenly puts a photo of Don Henley onscreen when Glenn Frey dies, she exclaims, “I’m sorry! I don’t know secular music!” Kayla also has a confusing one-night stand with a female co-worker (Kate McKinnon) that’s carelessly tossed in.

Lithgow coasts as Ailes, which isn’t entirely his fault. Most of his dialogue is barked orders and pervy whispers. Russell Crowe was much better in the role in Showtime’s “The Loudest Voice.” (Fox News is owned by a sister company of The Post.)

What “Bombshell” has going for it is a jaunty pace. The film by Jay Roach — the “Austin Powers” director who’s had rotten luck with dramas — clips along and is always watchable. But it misguidedly mimics other annoying, ripped-from-the-headlines movies, such as “The Big Short” and “Vice,” that rely on Elvis-impersonator acting, smug narration and quick cuts. Sometimes, you just want to see a tough topic taken seriously.