When is the star of a movie not really the star? When his co-star is coming off the biggest hit of the summer.Antonio Banderas knows firsthand. Though ostensibly the top-billed cast member of Once Upon a Time in Mexico, co-star Johnny Depp's mug is plastered all over the TV and newspaper spots; you would hardly know Banderas is in the movie. Mexico is the third in a trilogy of films about a hit man - all but the first starring Banderas in the lead role. It was produced in 2001, before Pirates of the Caribbean made the always vogue but rarely profitable Depp the hottest ticket in town. Now he's commercial gold, which is why Banderas has been relegated to second tier in his own film franchise. Hollywood being Hollywood, every studio exec in town will be rushing to sign Depp to their latest project, conveniently forgetting, as always, that all his movies before Pirates that didn't do squat at the box office. The folks who make decisions in Tinseltown get just as caught up in publicity buzz as everyone else. Their brains turn to mush at the possibility of landing an actor whose previous film just made North of $200 million. If they had longer memories (which they don't), they would recall that Depp's last four films combined didn't make $100 million. They would remember that audiences want to see their favorite stars in roles that they identify with that star. Sure, actors love to talk about "stretching" themselves in offbeat roles, but the moviegoing public doesn't want to hear it. I've worked on more box-office underachievers than I care to remember, starring the likes of Bruce Willis, Sean Connery, and, yes, Johnny Depp, that failed because they weren't playing the kinds of roles that clicked with people who buy tickets. Depp's wickedly funny turn as a pirate (think Dudley Moore as Arthur, but with a cooler costume) worked because folks smiled at the mere thought of him playing the part. Not so many people wanted to see his far more measured, virtuoso performance as a grieving, opium-addicted detective in From Hell. Or as author Hunter S. Thompson. Or as a bookish scientist. In fact, he even told me, long before the Pirates phenomenon, that he was surprised to still get any offers. Talented? Extremely. Bankable? No. Until now. We'll see how audiences respond when he returns to playing eccentric characters in the offbeat movies he relishes.