Hollywood -- Half the time you never know who you’re talking to -- that is, if you’re on the set of 20th Century-Fox TV’s “Planet of the Apes” series.   Most often this happens when the director approaches Galen, chimpanzee star of the series for CBS-TV, and begins to describe how he would like the next scene played. As he pauses for breath, instead of Roddy McDowall’s velvet English accent, the voice of a champion bronco buster emanates from behind the chimp’s mask to drawl: “I ain’t Roddy.”   An alimony dodger, a bank robber or a Public Enemy could hide out in costume among the “Apes” cast and live without fear of ever being served extradition papers. You just don’t know who is who until you’re (sic) really studied the becostumed actors’ traits or maybe recognised the voice.   Yet, attempting to identify a human behind an ape mask is most frustrating. For instance, the cowboy who plays Roddy’s double is Dave Rogers of Sonoma, Cal. After fracturing seven ribs, and both legs and arms twice, he decided to give up bull riding and stick to saddle broncs. When behind his mask (it’s truly an “appliance” which takes three hours to create each day), Dave is identical in height, weight and proportion to Roddy McDowall. Thus the mistaking of identity.   And Mark Lenard (who plays Urko, the gorilla) or Booth Colman (who portrays Zaius, the orangutan) have not yet been viewed in the real flesh by members of the crew. They know the ape—but not the man.   However you put it, with all the misidentification, it is extremely difficult for a poor mortal to “go ape!”