Allison Janney joins James Corden for his third ‘Carpool Karaoke Primetime Special’

Dressed in an all-black nun’s habit on a hot afternoon, Allison Janney emerges from her impromptu dressing room — a black tent on the parking lot of CBS Television Studios — and heads towards a group of celebrities headed by James Corden, host of the network’s “The Late Late Show.”

“Oh, lord,” she says, largely to herself. In a few moments, the recent Oscar winner (“I, Tonya”) and star of CBS’ “Mom” will join Corden, her “Mom” costar Anna Faris, Kunal Nayyar (“The Big Bang Theory”) and Iain Armitage (“Young Sheldon”) to invade the busy intersection of Beverly Boulevard and Genesee Avenue. Their objective: to stage a quickie number from “The Sound of Music.”

This segment of “Crosswalk! The Musical” will be a highlight of Corden’s third “Carpool Karaoke Primetime Special” airing tonight on CBS at 10 p.m. The hourlong special will open with a number performed to Bruno Mars’ “Finesse” and feature a new “Carpool Karaoke” session with Christina Aguilera.

Corden’s first two “Carpool Karaoke” prime-time outings earned Emmy awards for Outstanding Variety Special.

In the “Carpool Karaoke” sessions, Corden drives singers around and joins them in belting out their songs as they move through traffic, and many of the videos have gone viral. His “Carpool Karaoke” ride with Adele scored 179 million views on YouTube. The concept was picked up as a series by Apple TV in 2016.

His crosswalk musical segments have earned a loyal viewership as well.

In tonight’s special, Corden plays an egotistical musical director who puts the stars through their paces as they brave the active crosswalk.

Cameramen wearing bright orange vests are stationed at two corners, and a drone flies above traffic to capture the full scale of the production. Streets remain open, and the traffic lights aren’t controlled by producers. The musical numbers are timed to fit well within the duration of a red light, and LAPD officers stand nearby should any incidents occur.

“I can’t believe that it’s become such a huge production,” Corden said in a phone interview. “The first time we did it, it was me and six dancers doing ‘Grease.’ We had none of that stuff you saw — tents, scenery, props. The scale of it is crazy.”

“Action!” the director shouts into the hum of traffic, and Janney, dressed like Julie Andrews in “The Sound of Music” and wearing a blond wig, goes into the street and starts twirling as she sings, “The hills are alive with the sound of music.” Background dancers hold a satin green fabric backdrop of hills. Janney performs the number a few times, each time running hurriedly to the sidewalk before the light turns green.

Nayyar, Faris and Corden join Janney for other numbers, including “Do-Re-Mi” and “My Favorite Things.” Also dancing was Andrews’ granddaughter Hanna Schneider. Andrews’ daughter, Jennifer Edwards, watched the proceedings.

As the chaos unfolds on the street, cameramen walk between cars to capture drivers’ reactions, which range from amusement to confusion. Most motorists pull their phones out to record the performance. Some cautiously stop several feet away from the red light, prompting crew members to plea with them to move forward into camera range.

Said Corden, “The thing that never fails in the edits when you play it in front of a live audience is the cutaways to the drivers in the cars. That’s where the laughs are.”

After the music and the dancing stop, Corden, wearing a dress and blond wig, walks gracefully between the cars, prompting drivers to kiss his hand or hug him. He jumps into the passenger seat of one car and is almost driven away.

After all, traffic in Los Angeles waits for no one — even flash musicals.

alejandra.reyesvelarde@latimes.com

Twitter: @r_valejandra