If you entertained any illusions about Fidel Castro mellowing with age, they will be dispelled by "Fidel Castro Speaks," on Channel 7 from 10 to 11 p.m. tonight.

Barbara Walters recently went down to Cuba at Castro's invitation and came back with some interesting film of Castro moving about the island, plus some revealing thoughts about himself, the CIA, Angola and the future of Cuban-American relations.

Waletrs is not quite accurate when she states at the outset of the program that Castro had promised her that for the first time in many years, he would answer questions from an American television reporter in any area we wished to pursue."

In addition to talking to Bill Moyers - his two hour special on the CIA and Cuba will be on Channel 9 at 9 p.m. Friday - Castro also gave an interview to Fran Mankiewicz and Dan Rather which was on CBS Reports in the fall of 1974.

Castro obviously was intrigued by the idea of being interviewed by Walters. Though he speaks a little English at the beginning of the program as the two of them ride across the Bay of Pigs, and again at the end, the rest of his answers are in Spanish, simultaneously translated.

While fascinating, the interview must have been very tricky as it unfolded. Castro is a consummate actor and some of the things he says are so patently outrageous that the person interviewing him has no choice but to challenge him in a way that turns the interview into something resembling a debate.

The interview moves back and forth between the interview itself and Castro giving Walters a Cook's Tour of Cuba. The interview itself ranges from politics to Castro's personal life - is he married? Will he ever shave off his beard?

On the last questions, Castro is rather amusing. After explaining how the beard grew to be a symbol of his revolutionary movement, he tells Walters that when it grews completely white, he will then make the decision whether to tint it or shave it off.

On politics, he displays his probably well-founded paranoia about the CIA and what he claims have been 20 different CIA assassination plots against him, his admiration for the Soviet Union - "The freest of all countries" - and why he has Cuban troops in Angola.

The chief merit of the interview is that it demonstrates how tough Castro is going to be on the resumption of relations with the United States. At one point, he states that normalization will probably not come about before what he refers to as President Carter's "second term."

Yet as tough as the man is, he is also rather beguiling in the way that he uses television. He understands the medium and knows its power. And what comes through during this interview is that he is a very determined complicated piece of work.