We were among the lucky ones at a screening in Gloucester this week of “Nothing Can Hurt Me,’’ the new documentary about Big Star, the ’70s band featuring the late great Alex Chilton. The movie is excellent — it’s at the Brattle Theatre next weekend — but as an added bonus, the audience at the Cape Ann Community Cinema was treated to a post-screening hootenanny, a mini-concert of Big Star songs played by a group of local musicians, one of whom we recognized. Just as the band was about to play the Big Star classic “September Gurls,’’ Cars drummer David Robinson, who was in the crowd, was summoned to the stage. “I shouldn’t have missed practice,’’ the pony-tailed Rockport resident said sheepishly before sitting down to play. But Robinson, who was also a member of the Modern Lovers, managed just fine, demonstrating he’s still got chops.