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Speaking to Entertainment Weekly, Lucas said: "We know that for the fans it won't be the movie that they have been making in their minds for the last 19 years, so they all get bent out of shape.
"It's funny. I think it's funnier than the other ones, and it's exciting. So it's got all the stuff that all the other ones have. And Harrison's great in it."
The producer added that critics may round on the movie for not being the "Second Coming".
However, the Star Wars creator claimed he wasn't nervous about the film failing financially, but expressed doubts about whether or not it could beat Batman's box office success.
"Will it beat [The Dark Knight]? I don't know. But we'll certainly give Batman a run for their money."
Lucas admitted he didn't know if the Indiana Jones series of films would continue after this summer's franchise entry because director Steven Spielberg has got several other films lined up for the next few years.
Indiana Jones and the Kingdom of the Crystal Skull opens on May 22.
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