What do Sainsbury’s and Beyoncé have in common? Until recently, not very much. But, thanks to a stroke of reactive marketing genius, this unlikely association brought Sainsbury’s a surge of engagement on social media. What can marketers learn from it in order to capitalise on other brands’ publicity, turn a PR disaster into a PR triumph or score serious engagement from a single post? One thing is clear: when it comes to reactive marketing, fortune favours the fast.