“Bubbly” isn’t exactly the first word that comes to mind when one thinks of the regal Meryly Streep. And Lily Tomlin, with all her snarky cynicism, doesn’t particularly strike you as a giggly kind of gal. But then you meet them, and, well, maybe it’s a girl thing, but I wanted to hug them both.

Streep and Tomlin insist they’d never worked together, hadn’t even met until they showed up on the set of A Prairie Home Companion to play singing sisters Yolanda and Rhonda Johnson. But to see them together now, it’s clear that they developed an instant chummery: the wonderful spontaneity and seeming longtime comeraderie they evince onscreen as Yolanda and Rhonda has carried over to Meryl and Lily. They cavort and snicker like favorite sisters, and finish each other’s sentences in their eagerness to answer journalists’ questions.

“We sort of fell into our parts” as siblings, says Streep, and they did so with such a thrilling frisson of chemistry that they steal the movie with the sisters’ tales of their performing family and outrageous family legends (one involves a stolen donut), related mostly to Yolanda’s daughter, Lola (played by Lindsay Lohan). One amazing scene early in the film they shot, they say, in long extended takes, and its improv feel sprung from the fact that they couldn’t remember their lines over the 20-plus minutes of rolling cameras. “It just felt like playing music,” Streep explains, “lines would change from take to take.” “We’d try to amuse each other and surprise each other,” Tomlin says.

And they’re still doing it: