The 19-year-old star told The Knoxville News Sentinel that she brought two fancy dresses, but they didn't seem to appropriate for the Tuesday night screening benefiting the philanthropic organization Variety of Eastern Tennessee.
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Minutes before walking out before a crowd of hundreds of fans, she changed into a simple black-and-white strapless dress.
The appearance by Stewart and co-star, 17-year-old Taylor Lautner, raised a little more than $440,000 for the charity.
The screening followed a full-scale Hollywood premiere with lots of red-carpet glamour Monday night in Los Angeles. Fans waited in line for days to get a glimpse of the movie's young star.
Early reviews have not been great, but that hasn't stopped fans.
While it doesn't open until Friday, advance ticket sales are breaking records at online outlets in the U.S. and Canada. The original "Twilight" movie made $340 million atthe box-office.
European fans will get the first look atthe new movie. It opens today in France, Italy, Spain, Belgium, and Switzerland and on Thursday, Nov. 19, in Australia, New Zealand, Mexico, Argentina, Greece, Hungary, Kazakhstan, Georgia, Russia, Sweden, Thailand, and the Netherlands.