John Williams has scored some of Hollywood's biggest sellers--from the Star Wars trilogy to E.T. to Jurassic Park. He's no master of subtle orchestration; rather, he's a specialist in blockbuster sound, a facile author of the bombastic. A onetime conductor of the U.S. Air Force Band, Williams began his show-biz career as an arranger for Kraft Television Theater, and in the 70s he worked on a string of disaster movies; then, of course, he hooked up with George Lucas and Steven Spielberg, furnishing the brass-heavy fanfares for their escapist epics. But Williams is capable of surprises, as his tender, poignant music for Schindler's List--and his 14-year tenure as Boston Pops maestro--show. His sense of dramatic timing is impeccable, and he has a knack for playing to the audience. So when he makes his CSO debut this weekend he ought to know just how to pump up the brass section's volume for a medley of his greatest hits. Sunday, 7 PM, pavilion, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 728-4642.