Television's most potent new program brew this season is "Bewitched" starring Elizabeth Montgomery, Dick York and Agnes Moorehead (Channel 2 - ABC at 8:00 pm. Thursdays).

Right from the beginning , ratings for "Bewitched" boiled over the top of the cauldron. There was never any doubt that it would be one of the hit shows of the TV year.

"But, oh yes there was! I had some reservations about it," said Agnes Moorehead during a recent interview. "As a matter of fact, when they (the studio) first asked me to read the script I didn't think the show would sell."

"However, they made such an attractive offer that I accepted and when they came back and told me the show was sold, it surprised me very much. But then I am always surprised when anything is a big success. Can't tell you why. I just don't understand those sorts of things."

Miss Moorehead portrays Endora, the glamorous mother witch on "Bewitched." Her daughter is a lovely sorceress named Samantha (Miss Montgomery) who has married a mere mortal (York) and has vowed to kick the witchcraft habit.

Mother Endora disapproves not only of her son-in-law, but of her daughter's preoccupation with the mundane chores - cooking, scrubbing, laundry - that harass mortal housewives. Modern day witches, you know, summon up a little abbra kadrabra to cope with such tasks and then they teleport off to Paris for lunch and a fashion show.

"It is a charming program - pure entertainment - which is just what it is meant to be," said Miss Moorehead. "Perhaps that is why it is so successful, too. Viewers can sit and watch "Bewitched" and forget their problems with a smile. No analysts, no pill-pushers, no troubles - just entertainment."

Miss Moorehead has agreed to appear in only 8 or 10 of the episodes this season.

"I won't be seen in too many of them, but I think this is good because it keeps the part of Endora fresher," she noted.

"Of course I don't get as much money, but an actress has to use her head. Can't be greedy - the public sees too much of you and suddenly they don't want anymore."

Not much chance of that. Miss Moorehead is an outstanding actress of stage, movies and TV and her name has graced more credit sheets than she can recall.

Her latest movie release is "Hush Hush Sweet Charlotte" (with Bette Davis and Olivia DeHavilland) and she has already been named as a possible Oscar nominee for her dramatic work in that one.

Miss Moorehead's network biography reports she was born in Boston and moved to St. Louis with her family as a young girl. She received her early schooling here and, starting at the age of 10, spent summers performing at the Muny Opera.

"That was in the '20s" she said. "I was a very little girl and the Muny was my first professional experience. Naturally I felt there was no one on the stage but me."

"But, please! No research into my background! I don't think an actress should allow that - she should surround herself with mystery and glamour. How old is she? Well, that is nobody's business."