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Michelle Trachtenberg has proven herself the ideal go-to actress for naughty TV guest appearance: When she portrayed Sarah Michelle Gellar's "normal" little sister on Buffy, she had a penchant for kleptomania, on Law & Order: CI, she masterfully outwitted the cops and the media by staging an online kidnapping, and she perfected the demanding pop star character of Celeste on Six Feet Under. So it's no surprise that the actress's upcoming guest appearance on Gossip Girl will feature her wreaking havoc on fellow rehabber Serena. We're not sure possessing the natural ability to portray bitchy sidekicks is a blessing or a curse, but in Michelle's case, we always find her small-screen bad-girl characters are ones worth rooting for. [THR]