Fred Ebb once said the single most thrilling moment in musical theater was the entrance Bob Fosse choreographed for Chita Rivera in the original 1975 production of “Chicago.” He swore he could feel the audience’s heart thump when the legendary performer ascended on a bare platform from beneath the stage and stepped out like some dazzling goddess of the night into the eye of a white-hot spotlight.Looking back on that entrance, Rivera suspects the heart Ebb could hear thumping was hers. “It was always frightening,” she says, remembering how it felt to be “down in the bowels of the theater, beneath the pit, standing alone on that platform.” Everyone down below — including her co-star Gwen Verdon, poised to make her own drop-dead entrance — could feel the terror of the wait. She credits one old stagehand with helping to calm her nerves: “He was the sweetest man — he used to hold my hand.” The role of Velma, the murderous vamp that she played in “Chicago,” brought out an aspect of her personality she never knew was there. “It was a side of me,” Rivera says, “that was conniving, slick, smooth and a bit desperate — a person who’s not afraid to do what she has to do to survive.” In “Chicago,” she was partnered memorably with Verdon, who originated the role of Roxie Hart, and later with Liza Minnelli, who performed for six weeks when Verdon developed a throat ailment. “Liza had a different kind of passion and energy, and I would love to step aside and just watch her,” Rivera remembers. “She loved it so much, and she wanted to do it so badly, and she was just so happy to be there, she worked her tush off.” Playing Velma gave Rivera the chance to exercise her comic acting chops. “Nobody could write the humor like Freddy,” she says. “And the reason his songs are so funny is because they’re so smart and so honest.” It still makes her laugh out loud to think about the “audacity” of “Class,” the duet in which Velma and the foul-mouthed prison matron (played by Mary McCarty) drunkenly bemoan the sad state of modern manners. During the original run of “Chicago,” the aud’s reaction sometimes got the two performers in trouble. “That song always shocked the matinee ladies,” she says, remembering how their vocal disapproval of Ebb’s “naughty” language got louder and louder until it became a mighty chorus at the line: “Why is it everyone now is a pain in the aaaaass.” “At one matinee, the ‘tsk-tsk-tsk’ got so loud that Mary and I completely broke up and they had to pull us off the stage,” Rivera recalls. Whether it be performing with Verdon, Minnelli or McCarty, Rivera looks back at her “Chicago” experience as one of collaboration. “It was no singular sensation,” she says, making a sly reference to “A Chorus Line,” the tuner that left her show bereft of any Tony award in 1976. Fortunately, good things sometimes happen to good musicals and good people: After “Chicago,” Rivera went on to win her Tonys for other Kander & Ebb shows, “The Rink” and “Kiss of the Spider Woman.” And “Chicago” itself won six Tonys in 1997, including best musical revival.