For seven years now, Roddy McDowall, whose lengthy stage, screen and TV career includes prestigious and even award-winning roles, has been making a monkey of himself.   Monkeys.   The high-I.Q. chimp Cornelius in a 1968 feature film, “The Planet of the Apes,” he also engaged in simian-simulating in three of four sequels (only his voice was heard in “Beneath the Planet of the Apes” because he was otherwise committed when it was being made) as Cornelius’ kin, Caesar, and now has a third chimp identity - as Galen, presumably from the same family tree - in CBS’ “Planet of the Apes” series, Fridays at 8 P.M. (Channel 10). Roddy McDowall plays his third chimp in the “Planet of the Apes” series, Fridays at 8.   This week he can also be seen on CBS as Caesar-in “Conquest of the Planet of the Apes,” Thursday at 9 P.M.   He’s donned a monkey face on other occasions, too.   “One of my nicest experiences,” says the 46-year-old one-time child film star, “was last year in the Hollywood Bowl, at a benefit to raise money for the Shakespeare Theater.   The program consisted of musical numbers from shows, and Jean Simmons and I did ‘This Can’t Be Love.’   “She was all yellow chiffon and feathers, and I was in a tux — with the ape head!”   “Afterwards I did a similar thing on ‘The Carol Burnett Show.’”   Is he concerned about becoming “typecast?”   “Yes. No. I stopped worrying about that long ago.   “I do like very much playing roles in elaborate makeup. In the past I’ve done such things as Ariel in ‘The Tempest,’ which required fantastic makeup — very bizarre!   “I don’t like my face that much. No one really likes his own face. And to see yourself up there on a screen ten times actual size is awful!   “With makeup you could be 90 and still look the same.   “In this particular role, with makeup completely concealing your own features, the challenge to an actor is tremendous. You have to do it all with your eyes and your voice. It’s a real test.   “As Galen I’m supposed to be intellectual and sensitive. How do I convey intellectuality and sensitivity? I sound British!”   Traditionally, kid actors and animals are the most flagrant scene-stealers.   “Right!” McDowall agrees. “And I’ve been able to be both, so I’m home free!” However, he notes, even “animals” can be plagued by animals.   “Two films ago — no, three. Kim Hunter and I had a baby chimp, and they used a real one. He was adorable, but boy! was that a day! We all had to have shots!” *        *        *        *        * London-born Roddy has been a wage-earner since he was 5. At first a clothing model, he became an actor at 8 and crossed the Atlantic four years later for a film role.   His first Hollywood opus, “Man Hunt,” was no movie milestone, but his second, “How Green Was My Valley,” made him a star. Subsequent small-fry cinema stints included “My Friend Flicka,” with a title horse, and “Lassie Come Home,” with a title dog.   “Speaking of animal actors,” he says, “Lassie and Flicka should be proud of me!   “Actually, of about 80 pictures I was in, only four were with animals.”   The other two: “Thunderhead,” with a horse, and “Killer Shark,” with a fish.   Once a costar WITH an animal, now he’s a costar AS an animal.   “I have no hangups either way,” he says with a grin. “I don’t even mind when people say, ‘I loved you in ‘The Yearling’ — a picture I wasn’t in.”   Since “going ape” (he winces every time he hears the phrase), he has acquired something of an animal’s eye-view.   “While shooting the ‘Ape’ films,” he says, “I have become very annoyed with humans and the way they treat animals.   “Do I think there should be a society to protect animal actors? They’re better treated than the humans! I learned that in 1942 when I was doing ‘Flicka.’ After a day of work, that horse had a whole pasture and pond of its own on Stage 9 or whatever, so it wouldn’t have to go into cold water!   “What we need is a Society for the Prevention of Cruelty to Actors!” However, as Galen, he gladly subjects himself to a six-days-out-of-seven ordeal — reporting to makeup at 5 A.M. for the complex camouflaging that requires three hours a day to put on and an hour to take off.   The rubberized makeup is molded to his skin and he has to sit motionless while it’s being applied. After a day’s shooting, everything is discarded but the ears, and next day the makeup artists begin assembling Galen all over again. *        *        *        *        * While he’s on the set, Roddy has trouble with eating, drinking and “other functions.” He sips juices through a straw, eats yogurt with a long spoon, and smokes through an extra-long cigaret holder.   He insists on one day off per script, “to let my skin breathe.”   “During the first makeup test,” he recalls, “I nearly freaked out. The makeup is terribly uncomfortable, and so is sitting there so still for so long.   “The first day in Utah, for the first movie, my face was covered up for 22 hours. It was totally impossible!”   Because of allergy hazards, his face reportedly has been insured for $100,000.   Various precautions are taken. The epidermal “breathing time.” A double for long shots. Less burdensome makeup when he doesn’t have to speak. “Dubbing” to minimize the times he must speak. But it’s still an ordeal.   So why does he do it — and why did he agree to such week-after-week woes for his very first TV series?   Money, of course. He’s a part-owner, with a share in the profits from Halloween masks and other series-linked merchandise. And he’s liked the notion of using apes to underline humans’ follies ever since Arthur Jacobs broached it to him before the first “Planet” movie.   Also, he had been playing a succession of villains, and the extremely popular “Planet” films restored him to the “good guy” category. Good ape, anyway. (In his next film. “Funny Lady,” starring Barbra Streisand, he’s a sympathetic non-ape.)   He was getting a little tired of baddies, just as — earlier in his career — he grew tired of portraying juveniles.   “I played 14-year-olds,” he sighs, “until I was 23!”   He altered his “image” then by heading for Broadway. This time he broke a similarly monotonous mold by reversing the Tarzan the Ape Man formula becoming Cornelius-Caesar-Galen the Man Ape.   BELOW: The feature as it originally appeared.