Audio problem: In the plane, when the Captain says "I'm not too god, what do you think" his lips aren't moving.
Audio problem: When Lex is hiding the kryptonite in the chest, Superman walks towards him and says, "You diseased maniac! Did you really think you could hide it from me by encasing it in lead?" but his lips are not moving.
Audio problem: While flying with Superman, Lois falls down and starts to shout. When he grabs her, her mouth closes slightly and her expression changes, but the loud crying continues.
Answer: He wanted to see for himself if the stories were true. Some reporters tend to exaggerate the facts and if he had any other weaknesses. He couldn't be sure the kryptonite would work.
While I don't exactly disagree that Lex wanted to see for himself how invincible Superman is, I don't think that's the main reason why he did it. The bullets and fire were simply a charade to make it look like this was his security system. Keep in mind, he wanted Superman to enter his lair because the real trap was the Kryptonite that he had in the lead box.
jshy7979