''But we have to be invisible, if people are going to believe in the film,'' said Mr. Armstrong by telephone from his farm in Windsor near England's Pinewood studios. In Mr. Armstrong's case, it wasn't all that hard to be invisible: His resemblance to Harrison Ford ''is uncanny,'' he said. They first met while filming ''Raiders'' in Tunisia. Production people on the set kept mistaking Mr. Armstrong for Mr. Ford. ''People kept calling me 'Harrison,' and at first I couldn't figure out who they were talking about.'' He found out soon enough, and was doubling for Mr. Ford almost immediately. ''We're the same height (a touch over 6 feet), the same age (Mr. Ford is 41, Mr. Armstrong 37) and we trade our clothes and boots in all the scenes we do.''

Neither star nor stunt man will reveal how they accomplished the film's most technically complex physical sequence - Indiana Jones's 80-foot plummet from the roof of a Shanghai building through a series of canopies that break his fall, ''without cutting into the shot until he hits the last canopy,'' said Mr. Armstrong. ''It's a secret rig we devised,'' he said. He would neither confirm nor deny that wires were used to control his fall when he doubled for Mr. Ford in the sequence.

Actually, it isn't that easy to double for Harrison Ford: ''He's a very physical actor, a natural athlete, and he wants to do it all,'' said Mr. Armstrong. ''I say to him, 'H., we cannot afford to get you smashed up in this scene because we've got a whole crew here that needs to make a living.' And he says, 'Yes, you're right,' and does the scene anyway. He could have made a great stunt man himself.''

Mr. Ford yielded to one of Mr. Armstrong's pleas while filming another of the movie's complex and sustained stunt sequences - a roller-coaster ride chase scene on mine cars that ends in a lengthy fight to the death with the chief slave guard.

''At one point the guard throws me into the mine car,'' said Mr. Ford. ''And since I'd just come back from surgery, I had second thoughts about being the throwee.'' The turbanned chief guard in that scene was played by Pat Roach, the same actor who played Mr. Ford's Nazi opponent in the fight for the flying wing in ''Raiders of the Lost Ark.''

Why does Mr. Ford do so much stunt work himself? ''I do as much as possible because, with Indiana Jones, there are so many opportunities for characterization in the physical action. Really, that is the character - and in these moments of action you see Indiana Jones most clearly,'' he said.