One day in 1968, the year before she died, Judy Garland watched a television interview she had done with Dick Cavett. Noticing Cavett’s fidgeting, she wondered why she made everyone so uncomfortable. Because, a friend replied, no one knew if she was going to sing “Over the Rainbow” or open her veins. “Sometimes I do both,” Garland said, “at the same time.”

In “Judy,” Renée Zellweger plays a few variations on Garland near the end of her life: worried mother, needy lover, disaster, legend. The woman who remains out of sight, though, is the sadder, scarier Judy who threw a butcher knife at one of her children and threatened to jump out a window in front of another. Even so, Zellweger is solid in a movie that derives its force from its central mythic figure and your own Yellow Brick Road memories: the Hollywood supernova with the inner-child vaudevillian named Frances Ethel Gumm, a.k.a. Baby.

One of those biopics that tries to encapsulate the sweep and substance of a life by narrowing in on ostensibly representative moments, “Judy” concentrates on Garland’s bumpy, weeks-long engagement at the Talk of the Town, a London cabaret-restaurant where patrons sometime s threw breadsticks at the faltering talent. When Garland arrived in London in late December 1968, she was broke. She had been forced to sell her house in Los Angeles — fans and servants had been helping pay the bills — and had effectively become a vagabond, which is how Gerald Clarke describes her in his sympathetic book “Get Happy.”