With Justin Bieber in tow, Ariana Grande plays the Forum

Guess who showed up at Ariana Grande's concert on Wednesday night?

Only Ariana Grande knows what triggered a change in her attitude Wednesday night at the Forum.

Perhaps it was the knowledge that her surprise guests were soon to appear. Or maybe it was the sparkly hot pants (with matching stiletto heels) she’d just changed into. It might simply have been that bedtime had arrived for her youngest fans, leaving her free to act her age.

Whatever its cause, there was no missing the transformation that took place halfway through Grande’s concert, the first of several local dates on the 21-year-old pop phenom’s Honeymoon Tour. After 45 minutes or so of steady wheel-spinning, Grande suddenly shifted into higher gear, singing with more intensity and authority. And that’s where she stayed for the remainder of an ultimately impressive performance.

In a way the progression felt like a microcosm of Grande’s show business career, which has quickly moved from innocence to experience. She found low-level fame as a teen actor on the Nickelodeon series “Sam & Cat.” But two years ago she turned her attention to music, releasing a strong debut album, “Yours Truly,” that used shades of 1990s-style R&B to unsettle her squeaky-clean image.

Her hit 2014 follow-up, “My Everything,” was bolder, boosted by more vivid production and more grown-up collaborations with rappers including Big Sean and ASAP Ferg. Now, with “Sam & Cat” not long from memory, she’s already a credible arena act. (Grande is scheduled to play the Honda Center in Anaheim on Friday.)

At the Forum, she seemed to be channeling the self-restraint of her Nickelodeon days in edgeless renditions of “The Way,” her breakout single, and “Best Mistake,” which she performed while standing atop a rather sad-looking cloud. Even “Pink Champagne,” despite its bubbly promise, fell flat.

“This song is really old,” she said, and you could detect the note of apology in her voice.

Then, as though she’d fulfilled some kind of obligation to her past, Grande shook off the bland decorum and tore into her song “Tattooed Heart” like it was some kind of treasured soul classic. Sitting on a white grand piano, she sounded even more assured in the title track from “My Everything” and a ferocious cover of Whitney Houston’s “I Have Nothing,” for which she was accompanied by the song’s composer, David Foster.

The appearance by Foster, a principal architect of the modern power ballad, was gratifying for anyone curious about Grande’s creative influences. She’d sung his songs, she said, in every gay bar and karaoke bar she’d ever visited.

But the audience at the Forum hadn’t come with visions of a 65-year-old songwriter dancing in their heads. Fueled by rumors circulating on Twitter, they were hoping to glimpse Justin Bieber, who turned up unannounced at Grande’s concert last month in Miami as part of the goodwill tour he’s been on lately following a long stretch of tabloid troubles.

Sure enough, Bieber joined Grande on Wednesday for “Love Me Harder,” her risqué duet with the Weeknd. What’s more, he made it through the whole song, an improvement over his cameo in Miami, where he seemed to forget the lyrics.

“You did it!” Grande told him in a welcome flash of her sly sense of humor. “You remembered the words!”

Evidently bolstered by her encouragement, Bieber stuck around to sing “Where Are Ü Now,” his thumping collaboration with Skrillex and Diplo, and “As Long As You Love Me,” which Grande promptly stole with a series of high-powered vocal runs.

“I gotta get back in shape,” Bieber said, panting a bit, before he split, and his rawness only made Grande seem more precise as she closed the show with laser-guided versions of “Break Free” and “Problem.”

Bieber might consider asking her for guidance on how to leave a moment behind.

Twitter: @mikaelwood

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