From its opening shot of a dimming light bulb, the film defies expectations. According to 1930’s film grammar, a dimming light bulb signified only one thing: an execution. In the case of Sabotage, however, the bulb announces a widespread London blackout caused by the introduction of sand into a power station generator.
BTW, Don’t ask me what film “grammar” is. But from the quotation above, I think I can figure out what is being said here: some convention that existed during that time. Anyway (just in case you’ve been away with John Carter on Mars for the past few months), the light bulb does not reveal that the guy on the left is not Hitchcock (just someone pretending to be him; see also, below).