This is a question which requires a multiple answers:
1) "The Illusionist" is a prime example of how films used to be marketed. That is, they weren't hyped-up before they came out and gradual word-of-mouth, good or bad, is what really sells (or sinks) the film once it is released.
It is funny that when a movie has no huge marketing blitz we now assume the producers want it to fade quickly because it isn't that good... Which is true for a good percentage of films, I.E. Blade Trinity, where Wesley Snipes didn't make a single appearance (radio; TV; print) to promote the film... But it also comes down to just simple economics since TI is actually a low-budget, indepenedent film which was not made by a major studio if you take notice.
2) The second part which flows from number one is the producers are hoping Ed Norton's fanbase -- Along with Jessic Biel, Paul Giamatti and Ruffus Sewel -- Will carry the film because this film is going to be harder to market given it is a period piece and centers on a mix of various genres (romance; mystery; supernatural) instead of more mainstream, easily labeled elements like most big studio films do.
The film is one of those films which may be too "mainstream" for arthouse and indie lovers, but not "mainstream enough" for the general moviegoing public to really embrace as a "must see" film which is what audiences respond to these days (so-called event films), right or wrong.
3) The poster they chose to market this film is completely wrong in my opinion as this is a character study more than a film about magic, or other things.
This is what I meant about the film not being mainstream enough where unknowing audiences will go see the film thinking it is about magic and illusions, but instead is more of a character study grounded in the realism of 1900s Vienna, that has magic and some supernatural elements in it.
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Here is a collection of reviews for "The Illusionist" thus far:
REVIEWS
As you can see, they range from average to good... But they aren't great, or outstanding which is the real factor as to why this film may, or may not do that well when all is said and done.