IN the crazy and no doubt kinky history of casting calls, there
have surely been few like the one for ''Shortbus.'' It was all
about sex, but not because the actors were coming on to the
director. And not because the director was looking for big-name
hotties to lure audiences into theaters. He wanted people whose
sexual histories and hangups might become fodder for the
screenplay. He wanted erotically adventurous recruits for an
expedition to what he saw as an unexplored confluence of drama,
comedy and unblinking pornography.  And so, on a Web site for
what he cryptically called ''the sex film project,'' the
director, John Cameron Mitchell, solicited videotapes ''of no
longer than 10 minutes of you, the actor, talking about a true-
life sexual experience that was very important to you.'' Any
actor averse to having sex on camera, he added, need not apply.
Raphael Barker applied. On his tape he reminisced about a school
trip to Spain when he was 12, and about an older boy there who
taught him to masturbate. Soo