Jennifer Garner showed off her impressive vocal and physical stamina during Thursday’s opening-night perf of Broadway’s “Cyrano de Bergerac,” in which she plays paramour to Kevin Kline‘s long-nosed swashbuckler.What’d she do to prepare? “Alias,” she quipped at the after-party. So performing a long, 100-year-old play eight times a week is just as physically taxing as kicking bad-guy butt during her five-season stint on the ABC secret-agent skein? “No. ‘Alias’ was harder,” she said. But then, she added, she spent her college years training for the stage. When she started doing film and TV, she had to rein in the fill-the-auditorium acting she’d worked so hard to perfect. “Here it feels nice just to let loose!” she declared.