When Hollywood comes to Washington - which it is doing with more frequency than ever before - Stuart Neumann and David Siegel are often the people who introduce the West Coast filmmakers to the nation's capital.

"I've had people look at the Capitol and ask, 'Is that the White House?'" says Neumann, founder of Triumvirate Productions, an Arlington firm that specializes in finding locations for television and movie production companies.

So far the firm has pinpointed Washington locations for such movies as "Three Days of the Condor," "All the President's Men," "Eleanor and Franklin," "Washington Behind Closed Doors," "The Other Side of Midnight" and "F.I.S.T."

Remember the parking garage in which Deep Throat shared his secrets with Robert Redford, who portrayed Post reporter Bob Woodward in "All The President's Men"? Actually, it was a garage in Rosslyn, just down Fort Myer drive from the offices of the film production company. (The interior garage scenes, however, were filmed in Hollywood.) In Sylvester Stallone's just-released film "F.I.S.T.," a climactic scene has the head of a trucker's union emerging a broken man from hearings in the House's Russell Office Building. Neumann can't remember how many letters he had to write to secure permission to film the scene - which included hundreds of extras clogging the Russell Building rotunda to cheer their leader.

"We try to convince authorities that we are showing Washington in a positive light," says Neumann, a 24-year-old Virginia native who parlayed a youthful interest in moviemaking into his present career. Still, he works in a political town where everyone doesn't share Hollywood's perceptions. In "Washington Behind Closed Doors," Neumann said he encountered some hostility at identifying the Labor Department headquarters as the reelection offices of a candidate easily identified as a Nixon-like character. The series was filmed here between Gerald Ford's defeat and Jimmy Carter's inauguration, which meant some Republican political appointees were still in office, so egos had to be treated carefully. Andrews Air Force Base withdrew permission to film there after the show's producer gave a lengthly newspaper interview about the series.

For "F.I.S.T.," Siegel had to locate fifty tractor trucks from around the country, all of which had to be more than twenty-nine years old. In the Washington area he found 100 antique cars, some laundry trucks and taxicabs.For their efforts, Neumann and Siegel charge around $200 a day, unless they've negotiated a package price beforehand.

Just-completed or upcoming projects: Charles Colson's "Born Again" ("We and the director were the only three Jews on the set," says Neumann), a film version of the musical "Hair," a Sophia Loren picture called "Firepower," Alan Alda's movie "The Senator," Sean Connery's "Meteor," and a couple of television productions.