Jennifer Garner is letting fans in on some behind-the-scenes details from the set of one of her most classic roles.During a recent Q&A session with theSkimm, the actress revealed why one scene almost made her 13 Going On 30 costar, Mark Ruffalo, quit.“We started to learn the dance,” she said, recalling the big sequence in the movie where a bunch of party-goers perform Michael Jackson’s ”Thriller” choreography.