I believe at this time Kirsten Dunst has no intention of doing another Spiderman film. That said, I believe a lot could change in the next two to three years, which is how long it would take to begin work on a fourth film.
After Spiderman came out 2002, Kirsten was Hollywood's "it girl". I even saw her referred to in that manner during an SNL skit that year. But since that time her only "hit" movie was Spiderman 2. Her other movies (Mona Lisa Smiles, Eternal Sunshine, Wimbeldon, Elizabethtown) haven't performed all that well. Kirsten Dunst isn't a box office draw. People don't go to a movie just because she's in it. I don't expect that trend to change when Marie Antoinette comes out this fall, an 18th century period piece set in France.
She's also not a media darling (Lindsey, Paris, the Jessica's --Alba and Simpson, Brittany, etc). You don't read a lot about her in the press. This isn't a bad thing for the average person. But, in Hollywood image is everything. Every Lindsey Lohan movie since the Freaky Friday has grossed less than the one before it, but Lindsey is still considered "an it girl". If Kirsten's movies aren't big hits and she doesn't capture the public's imagination, her star will start to dim and she won't be as in demand as she once was. The shelf life for an actress in Hollywood is usually pretty short.
If she's cool with her current status I doubt she'll appear in another Spidey flick. If she wants a big check and/or a big hit to keep her before the public's eye, I wouldn't be surprised if she returned for a fourth film.....sepecially if Maguire and Raimi sign on.
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