Joni Mitchell was having trouble singing "For Free" with a straight face. When she got to, "I've got a black limousine," she added that she really had three-and a bus and Winnebago. When she sang, "I've got two gentlemen escorting me to the hall," she added that she now had sixteen and then cracked up laughing, finally blurting out an "Oh, shit." She plowed back into the song and finished it with a smile, but when anything slowed her down-an out of tune guitar, a touch of feedback, or just too much time between songs-she thought out loud that everything had gone perfectly until she'd said "shit" on stage. Joni Mitchell, a puritan? Well, Maybe just a little bit.

Her puritanism was hardly so striking as the awareness of it she was showing in her Music Hall concert. In fact, although her lyrics have always revealed a remarkable degree of introspection, in concert one is inevitably awed by the level of concentration lurking beneath nearly every line and gesture. If she "… really [doesn't] know life at all," it isn't from lack of trying.

She's come a long way from the dainty artiness of her first album, Joni Mitchell (from which she surprisingly took "Cactus Tree" to open the second half of the concert). She started out writing art songs and has by now invented her own genres, carrying the confessional and autobiographical lyric to unprecedented extremes. In the process, the musician whose pull over her special audience has been so irresistible has also become extraordinarily popular-so much so that Court and Spark is outselling Planet Waves all over this town and many others; proof that there is still justice in the world.

Like Dylan in his prime, Joni Mitchell changes with every new release and every new tour. When she performed at the Music Hall two years ago she seemed to be suffering from a lingering case of stage fright. She didn't want to think of herself as a performer. Now she no longer looks like she needs a manager to push her on stage; instead she pushes herself out there. Performing is no longer something she has to do, but something she wants to master. On both "Woodstock" and "Twisted" she stood up without any props (guitar or dulcimer) and simply rocked out. And she appeared for the second half of the show in a glamorous backless dress, not just to emphasize her concept of femininity, but also to prove that she is an entertainer who has learned to not just cope with, but actively enjoy, that role.

The most important change wasn't so immediately personal. Tom Scott and the L.A. Express, a fine band in their own right, add an entirely new dimension to her concerts by accompanying her for two-thirds of her presentation. The presence of five fine musicians to share the burdens of the stage seems to relax her, although she hasn't lost the ingratiating giggle she uses to hide her nervousness between songs.

Calmer herself, her music is calming too. She wanders over her repertoire, but focuses the show around no less than eight songs from the new album, all of them close to their recorded versions, with Scott's hornwork supplying the breadth of sound that was so innovative, for her, on Court and Spark. Just as with the album, the increase in instrumentation sometimes produces a greater degree of intimacy than when she performs solo.

Her voice was stronger but rougher than on records. She drew even more power from "A Case of You," done with only dulcimer accompaniment, on stage than she did on Blue. But she could also so understate "You Turn Me On I'm A Radio" that she seduced the listener just as skillfully as she tries to seduce the man of the song-with an extraordinary combination of sincerity and slyness.

On both "Woodstock" and "Twisted" she bordered on jazz style, with fine results. At the end of "You Turn Me On" she used her voice like an instrument and did a fine call and response with Scott's guitarist. On "Car on a Hill," "Same Situation" and "Just Like This Train" she reproduced the effects of her new album with only slight departures. But just as she expanded some songs by adding the band, she subtracted it from "People's Parties," giving it an even more melancholy feeling, and did "Big Yellow Taxi" as a solo, when I would have guessed she would use the group. She did her greatest variations on her original vocals on the older songs, especially "A Case of You" and "Both Sides Now," which closed the show. But unlike Rick Danko, who seemed, in concert, to vary his singing of old Band tunes in a haphazard way (probably out of mere boredom), Joni Mitchell used her impatience to enhance her interpretations. She seemed to struggle for a new level of intensity in an honest effort to renew each song's meaning.

She told an honest but funny story about how she came to write "For the Roses," her definitive commentary about the pains that result from so many people wanting so much from her. It took just a touch of the sting out of the song and reminded us all that she is still here performing, so it can't be all that bad. Her stratagem worked because it made us deal with the music on two levels: as a confession and as entertainment.

The confession breaks itself down into both sides now. She understands the extremes so perfectly that she is caught between them. She is in the middle of being free and being committed, and struggling to be both. She is in the middle of being a rocker and an art singer, and struggling to be both.