POP MUSIC REVIEW : Waking Up Isn’t Hard to Do for Cowboy Junkies : Lead singer Margo Timmins and the band play against type and show they can rock at Coach House in San Juan Capistrano.

It was one of the cagiest, most startling musical moments we’re apt to see for a long while.

There was Cowboy Junkies singer Margo Timmins, a shrinking violet of a performer so fragile and retiring that she appears most comfortable communing with herself in an enervated, inward murmur. But, there she was, smiling and singing with something approaching boldness as she hammered home a famous line borrowed from Lou Reed: “Me, I’m in a rock ‘n’ roll band.”

Cowboy Junkies had rendered that line before, in the subdued, dream-state cover of Reed’s “Sweet Jane” that appeared on the band’s 1988 breakthrough album, “The Trinity Session.” But Wednesday night at the Coach House, Timmins and company cannily played against type and sounded a wake-up call. They made “Sweet Jane” a declaration that they are, indeed, a rock ‘n’ roll band, and not just the purveyors of a preternatural hush that they were on “Trinity.”

“Sweet Jane,” which turned up in the first of two encores of a warmly received show, began cool and glistening, with a mellow, rounded bass current that made it more like Reed’s “Walk on the Wild Side” than the “Sweet Jane” he originally recorded on the Velvet Underground’s sublime 1970 album, “Loaded.” But with that line, “Me, I’m in a rock ‘n’ roll band,” lights suddenly flashed, amps suddenly cranked, and the band burst delightedly into the transcendently choppy three-chord grind that defines “Sweet Jane.” They rocked that riff for all it was worth--more satisfyingly, in fact, than Reed himself had done last week at the Greek Theatre. And Margo Timmins, that wilting wisp of a lass, was out there projecting, in her own demure way, a real rocker’s spirit and self-assertion.

It made one sit up and think, “By God, they really are a rock ‘n’ roll band.”

Well, intermittently.

Instrumentally, at least, Cowboy Junkies made a consistently strong case that they can rock with assurance. Drummer Peter Timmins (one of the band’s three Timmins siblings, along with Margo and rhythm guitarist Michael) played with a firmness and energy that hinted he’d about had it with the somnambulism of “Trinity” and its only slightly less sleepy 1990 successor, “The Caution Horses.” The drummer played as if thirsting for motion and force--two elements much more in evidence on the Junkies’ latest album, “Black Eyed Man.” Ken Myhr’s lead guitar provided deft accents for the country-inflected mid-tempo numbers that characterize most of the new material; when given a chance to rock, he mustered sting, bite, heft and even a capacity for dissonance.

Before “Sweet Jane,” Myhr and keyboards player Spencer Evans had powered Cowboy Junkies through an almost psychedelic swarm of sound in the instrumental finale of “Murder, Tonight, in the Trailer Park.”

That was rock, all right. But two key members of Cowboy Junkies continue to be particularly ill-suited to be in a rock ‘n’ roll band. Michael Timmins spent the entire 100-minute affair seated and hunched over his guitar. As long as he keeps coming up with songs as rich in imagery and narrative as the ones on “Black Eyed Man,” Timmins, who writes the band’s songs, can concertize from a hammock, for all that it matters. But Margo has to shed the soft cocoon that too often smothered her singing.

The show offered some clear signs that she can do it. Timmins may not have screamed bloody murder in “Murder, Tonight . . . ,” but she sounded an alarm that would have wakened the neighbors to the criminal doings detailed in the song. On “Misguided Angel,” an audience favorite from “The Trinity Session,” Timmins showed she could be assertive in a quieter setting--the wistful maiden showing some spine. A show-closing cover of Bob Dylan’s “If You Gotta Go, Go Now,” offered a firm, if not quite raucous, vocal that stood up to the band’s raw, muscular attack. During “Lost My Driving Wheel,” a deep, lonesome lament that was part of an impressive troika of good, as-yet unrecorded new songs the band strung together at mid-set, Cowboy Junkies built a steadily rising instrumental surge. Timmins caught that wave and rode it firmly and gracefully.

She also stepped gingerly out of her shell to serve as a shy but interesting hostess capable of drawing in the audience with brief song introductions that were sometimes quietly humorous.

Timmins’ moments of strength were fleeting, though. Too often, she retreated to her customary murmur and allowed the band to wash her voice away. The story-songs on “Black Eyed Man” demand a confident, commanding actress-narrator who can make the characters vivid and bring their passions to life. Lapsing repeatedly into the inwardness that obviously is her natural bent, Timmins didn’t come close to doing them justice. In fact, confronted with the band’s new instrumental assertiveness, she seldom managed even to make the words intelligible. That introversion kept her dusky-but-tremulous vocal blend from taking the prominent place it must assume if Cowboy Junkies is to fulfill its promise as a live band.

There’s no way that Timmins will ever be a stage-bounding, eardrum-blasting, eye-riveting firebrand like Maria McKee. But maybe she has it in her to evolve from too-fragile maidenhood toward the wistful but nevertheless sturdy presence and vocal bearing of an Emmylou Harris.