What's Jack Nicholson's secret? Maybe it's the eyebrows, hovering like ironic quotation marks over every line reading. Maybe it's the hooded eyes, which hold the threat of danger or the promise of joviality -- you're never sure which. Same with that sharklike grin. Or maybe it's the voice, which has evolved over the years from a thin sneer to a deep rumble, but is always precisely calibrated to provoke a reaction. Put them all together, and they say: "I am a man to be reckoned with. Ignore me at your peril." Nicholson, who turns 75 on April 22, is often criticized for relying on his bag of tricks, for just showing up and doing Jack Nicholson (though indeed, he often seems to have been hired precisely for that purpose). But he's also capable of burrowing deep into a character, finding his wounded heart, and revealing the ugly truth without fear or vanity.