A typical Jerry Lewis movie. As with most of his, if you don't like Lewis you won't like the movie, but also as with most of his films, it's packed with funny comedy that's mostly timeless and often imitated.
The cast includes Brian Donlevy (Cowboy, The Quatermass Experiment, Beau Geste, etc.), Howard McNear (Floyd the barber on The Andy Griffith Show), and Joe Besser (The Three Stooges, The Abbott & Costello Show).
The plot's not; just a flimsy frame for Jerry's shenanigans, of which there's enough to fill 90 minutes with laughs, plus a couple cute scenes with puppets. Jerry's hired ostensibly as an errand boy for a movie studio, but actually to spy for the upper management. He get's into plenty of slapstick trouble on the job but never accomplishes any spying.
But the comedy is not all slapstick by any means; much of it is poking fun at movie directors and supervisors, and other screwball comedy. One of my favorite bits is when three kids are ordering candy where Jerry's filling in at the studio cafeteria, and they send him up the same high ladder 3 times for different orders from the same giant glass jar of jelly beans. There's also a great basketball scene with the Dover Basketeers. But wait, there's more, including a cameo appearance by the Cartwrights of Bonanza, Lorne Greene, Michael Landon, Pernell Roberts, and Dan Blocker.
My favorite Jerry Lewis movies are "The Delicate Delinquent," "Way... Way Out," (which doesn't seem to be available anywhere) and of course "The Nutty Professor" (nothing against Eddy Murphy, but he's no Jerry Lewis). I think The Errand Boy is almost as good as these.