The thing, I guess, is that you don't really believe them. You get the impression that, no matter how well they read their lines, they are only playing a part.

Yesterday, it being Stacy Keach's turn to play, television's "Mike Hammer" told a House Narcotics Committee hearing what a terrible thing the seductive cocaine turns out to be, how glad he is that he finally came to his senses (after half a year in a Reading gaol) and how fervently he hopes the rest of us will learn from his horrible example.

"There is no greater imprisonment than that of being dependent on any chemical substance for one's own existence," Keach said. "By far the worst form of incarceration is to be trapped within one's own powerlessness to help oneself."

There's not a thing wrong with those words. One might have heard similar thoughts from John Lucas, the basketball pro; Jim Vance, the Washington TV anchor, or from any number of big-time entertainers and athletes. But you never seem to hear them say anything until their careers -- and their money -- are publicly in jeopardy. Then they see the light.

Carl Eller, the ex-Minnesota Viking lineman who also testified at yesterday's hearings (along with Bernice Carrington, a former bank employee who said she let cocaine drive her to robbery and prostitution), said that toward the end of his career he was spending almost all of his salary -- some $2,000 a week -- on cocaine.

The tales of wasted money, wasted careers, wasted lives are uniformly sad, and some of these sad tales must come from people who truly have reformed. But there is always the suspicion that some of them are going public less in an effort to save your life than to reclaim their own. You keep wondering why they never said anything before they found themselves in deep trouble.

Maybe the reason lies in the nature of the addiction. Maybe you don't know you've got the habit until circumstances force you to try to give it up. As Keach put it, he "foolishly and blindly refused to abandon the notion that I could take it or leave it" until he got busted in Heathrow Airport. That seems to be the standard line, though it's hard to imagine why spending up to $250 a day on coke (as Keach said he was doing) or $2,000 a week (as Eller said he was) would give you just a clue that maybe it, and not you, was in control.

But sincerity isn't the only question that these recitals bring to mind. Perhaps the more serious question is: What good do they do?

The problem is clear enough. As Charles Rangel (D-N.Y.), chairman of the House Select Committee on Narcotics Abuse and Control, said yesterday, cocaine claimed 456 American lives last year -- up from 129 in 1979- 80. "A hundred tons of this white poison" will be smuggled into the United States this year, he said, noting that the quality of coke has gone up while the price has come down. It's a problem that threatens to keep growing.

And yet nobody seems to have any idea of how to retard that growth. Interdiction of supply clearly hasn't worked, and drug-abuse education, the route recommended by yesterday's star witnesses, hardly seems more promising.

Just last week, Jesse Jackson asked a crowd of some 500 youngsters how many of them had experimented with illegal drugs. A good number admitted they had. Then he asked those to stand who personally knew people who were either in jail or dead as a result of illegal drugs. About half the audience stood.

The point: If teen-agers scrounging for summer jobs know what the drugs can do to you, surely the stars of entertainment and sports know. It's hard to see how education can be the solution if ignorance is not the problem.