[8] She played opposite many notable actors such as Paul Muni, Richard Barthelmess, Franchot Tone, Joe E. Brown, and Warren William.

[8] The studio executives were also unhappy with her political activism, "offering financial support to striking lettuce workers in California's Imperial Valley and signing on as an early member of the newly formed Screen Actors Guild (SAG).... Muir marched in the union's first Labor Day parade, held organizing meetings at her home, and made phone calls to secure picketers ahead of the union's first possible strike.

"[9] But perhaps what most annoyed Muir's employers was her tendency to ask too many questions, as her biographer Martha Fischer notes: This new girl immediately made a nuisance of herself around the studio, strolling onto soundstages and interrogating everyone from electricians to lighting directors about their work.

In Muir's own telling, she quickly earned the nickname "the studio pest," believing no one too important to be questioned, and never doubting her right to ask.

When she was cast in films, Muir questioned her directors, seeking explanations for their orders rather than simply doing as she was told.

[9] She eventually returned to Hollywood for a few more films, the last being The Constant Nymph (1943), acting alongside Joan Fontaine.

Although she obtained two short-lived Broadway jobs in the ensuing years—including a role in Tenting Tonight in 1947[6]—she concentrated on raising her three children.

[14] The apparent reason for the listing was that she once held a six-month membership in the Congress of American Women, which federal authorities had labeled a subversive group.

[12] In an attempt to remove herself from the blacklist, Muir volunteered to appear before the House Un-American Activities Committee (HUAC).

Martha Fischer writes: Afterward, she instantly regretted her choice, not because there was no clearance forthcoming – though there wasn't – but rather because it made her feel like "a craven turncoat against all my principles."

[9]After the blacklist finally eased for Muir, she resumed television acting in 1958 when she appeared in an episode of NBC Matinee Theater.