The bulk of this comedy-adventure flick chronicles the misadventures of small-time loser Jerry Welbach (Brad Pitt), who owes the mob one last favor. The “Mexican” in question is a priceless antique pistol that Pitt’s mob boss wants him to recover from down Mexico way. The only problem is that when his girlfriend Samantha (played by Julia Roberts) finds out that he’s still involved with the mob, she splits for Las Vegas, alone. Jerry’s search for the pistol takes him off Mexico’s beaten paths into the scenic highlands as he stumbles from one predicament to the next, only to discover that the pistol carries a legendary curse. Could this account for his bad luck? While Jerry is likable, he’s an unfocused type, so mob hit man Leroy (The Sopranos’ James Gandolfini) takes Samantha hostage in order to keep Jerry’s mind on his work. Since most of Roberts’ scenes are actually spent talking about Jerry rather than interacting with him, the “stars” don’t share much screen time. In other words, if you’re expecting a love story between stars Pitt and Roberts, you can forget it. See “Opening Friday” for theaters and times.