Fiona Apple leapt into the air, landed and then banged a giant drum with a mallet.

That’s about as happy as Apple — pop’s princess of pain with her raw, self-possessed, baldly confessional songs — gets.

“That is exactly what I worked 20 years for,” she told guitarist Blake Mills, her duo partner on their Anything We Want Tour.

Well, before you could name her 1990s MTV classics “Criminal” and “Shadowboxer,” Apple was downbeat Monday at the O’Shaughnessy auditorium at St. Catherine University in St. Paul. “I’m not crazy,” the Grammy winner told the near-capacity crowd. “I’m a little daffy.”

And a whole lot less innocent than when she first played at St. Kate’s as a 20-year-old in 1997. Apple has made only three more albums since then, but she has become one of pop’s most intense and captivating but challenging artists. Her concerts are like primal therapy for her — except fans are paying $60 for a session that is tense, harrowing and ultimately liberating. Purging in the name of art, you might call it.

On Monday, Apple’s moods seemed to change as often as Beyoncé changes outfits in concerts — about every other song. That’s partly because Mills’ periodic country-folk plaints were perversely optimistic compared with the plunging into her tortured psyche.

Part of Apple’s appeal is that she can be as volatile as she is vulnerable, and that brings the excitement of uncertainty to her shows. She has been known to respond, for better or worse, to shout-outs from fans. Luckily, no one at St. Kate’s ventured anything more than “I love you.” Even when things seemed to devolve. Like when Apple started a song and then quickly asked for a “do-over” midway through the first verse. Or when she momentarily freaked out because she couldn’t read the set list.

But Mills, 27, became the 36-year-old Apple’s security blanket. The well-traveled Los Angeles guitarist, who has played with Lucinda Williams, Dawes and Jenny Lewis, opened Apple’s 2012 tour and played in her band then, as well. She’s clearly comfortable with him.

At one point Monday, Mills said they were about to play his favorite Apple song. Bassist Sebastian Steinberg, who also was in her band last year, echoed that sentiment, which seemed to relax the always wired Apple. As the night wore on, Mills said two other songs were his favorites. The audience laughed, and Apple almost did, too.

The 95-minute gig felt casual, organic and a little experimental. There was a hiss in the sound system that was so constant — and noticed by both Mills and Apple — that it made you wonder if it received union scale for its percussive participation.

Even though Apple eschewed her hits and played little piano, there were magical moments that lived up to her magnificent 2012 concert at the Orpheum: the soulful purge “Regret,” the jittery “Dull Tool” and a chilling cover of Conway Twitty’s “It’s Only Make Believe.”

In the end, the erratic Apple lived up to the words she’d scrawled on a blackboard at the beginning of the evening: “Teach me how to be free.”