Juno (2007): Sassy. Dry witty dialogue delivered with a mixture of deadpan and playful expressivity stretched to bursting by Ellen Page as Juno. Allison Janney and J.K. Simmons also play likeable characters winningly. Since it's about a 16 year old realising she's pregnant you might not seek this film out regardless of how upbeat it is. Try. Here's the OED on what it means to be sassy (and they should know): "Impudent, saucy, ‘cheeky’; outspoken, provocative; conceited, pretentious; self-assured, spirited, bold; vigorous, lively; stylish, ‘chic’".