Outside trappings aside, though, this is still a television program and not a trial. One distinction is the choice of witnesses; they are summoned not by subpoena but by invitation from the producers, London Weekend Television. Some witnesses are practiced. They testified before the Warren Commission or the House committee. Some grin; they know this is make-believe. Some have special cases to plead.

We hear, for example, from one of Mr. Oswald's old comrades in the Marines. He says he and his family had to flee the country because the F.B.I. wanted to kill him. Mr. Bugliosi dismisses this; he lets the witness go. We see Mr. Spence rising from his chair then, torn by inner agony. It is apparently a great burden for him to speak.

Finally he says, ''You were actually shot in the shoulder, weren't you?''

The witness nods; score a point for conspiracies. The next witness is Ruth Paine, who befriended Marina Oswald, Mr. Oswald's Russian wife. After the assassination, many reporters noted that Mrs. Paine seemed to be ingenuous and modest. She still seems that way. One wonders what prompted her to appear on television.

Mr. Spence asks if it was a ''coincidence'' that Mrs. Paine studied Russian, a ''coincidence'' that she befriended Mrs. Oswald, a ''coincidence'' that she directed Mr. Oswald to a job at the Texas School Book Depository, a ''coincidence'' that the Mannlicher-Carcano rifle was stored in her garage?

It is a tricky line of questioning. On one hand, all those ''coincidences'' indicate a plot, secret purposes, great forces that move in the dark. Obviously, Mr. Oswald was a pawn. On the other hand, Mrs. Paine appears genuinely bewildered, and Mr. Spence risks looking like a bully.

Neatly, however, he escapes the dilemma. ''Now you know,'' he says, ''how Lee would feel.''

We are watching a master defense attorney here. Has he bullied Mrs. Paine? That's inconsequential; think of how the Government defamed Mr. Oswald. At the same time, the idea of conspiracy has been advanced. Mrs. Paine may seem like an unlikely conspirator, but the plot is so vast and dark - Mr. Spence has mentioned both the Pentagon and K.G.B. - that who can tell?