She may have been TV's wise-cracking Flo of seasons past, but the new role for Polly Holliday, 44, was no laughing matter. Two days after Eileen Brennan, 48, was struck by a car, Holliday was asked to step into Brennan's CBS sitcom, Private Benjamin, while the critically injured actress recovered. "I hated to get a job at the expense of someone's misfortune," says Polly, who had met Eileen only once previously. "But a lot of other people's jobs were at stake. I didn't want to see the show fold."

While Holliday prepared for her hastily written part as Major Amanda Lee Allen, a "gung-ho Army woman with the military in her blood," Brennan began what could be several months of recuperation at Daniel Freeman Marina Hospital in Marina del Rey. She already has undergone two operations to repair injuries that include two badly broken legs and facial fractures. Her sisters, Gerry Mangurian and Kathleen Howard, rallied quickly to her side, and Brennan's ex-husband, photographer-poet David Lampson, 42, flew down from British Columbia to care for their sons, Patrick, 9, and Sam, 11.

"Her spirits are great," says sister Gerry. "She keeps everybody in the intensive care unit laughing. She's very concerned about the man who was driving the car that hit her [29-year-old grocery clerk Ronald Guajardo] and wants to talk to him. He's been wonderful. He came to the hospital and sent her flowers. It's just an unfortunate accident, it could happen to anyone." One of her first visitors was Goldie Hawn, 36. Brennan and Hawn had been dining at a Venice, Calif. restaurant and were saying goodbye when Eileen walked into a street darkened by a power failure and was struck.

At the time of the accident Warner Bros, had completed shooting 11 of 13 contracted episodes for Private Benjamin's second year, and further segments had to be produced almost without interruption. Holliday, who was visiting L.A. at the time, surmises she was tapped "because they know I can come in and get it together quickly." As the career military officer who makes life miserable for Lorna Patterson, who plays the title role, Polly will eschew Flo's strawberry-blond bouffant; in its place are gray wigs cropped regulation fashion "just below the ears." For story-line purposes, Major Allen will be assigned to temporary duty at "Fort Bradley." Her first show will air in late December.

Polly seems ready for her new command. After Flo was canceled in July 1981, she admits, "I was tired and went home to mother's in Alabama to rest and vegetate. Then I went to New York and moved into a new apartment. I've also been writing a little, just scribblings." Filling in for an established star poses a new—and delicate—challenge. "When you work together in a series you become family," explains co-star Patterson. "I felt strange when they recast Eileen's part, but the show must go on. It's awkward for all of us, Polly included, but she's an incredible professional and I thank her for stepping in. I want to keep our show alive so Eileen can come back." That, too, is what Polly Holliday wants. As Major Allen, her mission is to hold the fort.