By Patrick Kevin Day, Times Staff Writer Hollywood has a lot of stars and an ever decreasing number of character-driven stories to cast them in. What's an actor to do? Many take what they can get and film cameos. And during no time is the cameo more prevalent than in the summer months, when high-concept franchises throw money around on big effects and winking walk-ons from familiar faces. In Hollywood, big franchises enjoy fist-bumping other franchises. Look no farther than David Hasselhoff in his time-honored "Baywatch" trunks as he takes a dip with SpongeBob in "The SpongeBob SquarePants Movie." Used wisely, the cameo can add cachet and novelty to a movie: audiences loved Sean Connery's last-minute appearance in "Robin Hood: Prince of Thieves" in 1991. But done without panache, the cameo can become nothing more than a cheap gag: Consider Kevin Smith's "Jay and Silent Bob Strike Back" in 2001. This summer has had a high number of cameos and we're only at the halfway point.