The Reagan administration says it wants to get its act together before deciding if it wants to get involved in nuclear arms talks, and what kind, and when, and with whom.

One act it's still working on is the one over at the Arms Control and Disarmament Agency.

First there was the question of who would play the lead. That was settled when Eugene V. Rostow, Yale law professor and chairman of the Committee on the Present Danger, was picked as director. The choice threatened to run into trouble from some conservatives who preferred retired Army Lt. Gen. Edward Rowny, who was the Joint Chiefs of Staff representative to the strategic arms limitations talks (SALT) for six years, but the word came that Rowny, too, would get a meaty role -- that of chief negotiator for SALT.

Then there were some problems with stage directions. Rowny wanted his own piece of turf outside the purview of ACDA. Senate Foreign Relations Committee Chairman Charles Percy (R-Ill.) wasn't crazy about the scene-stealing potential of such an arrangement, and he made it clear to the White House that he thought the SALT negotiator should be an ACDA executive.

That problem apparently was ironed out in a high-level meeting earlier this week in which Rowny agreed to accept a suitable position within ACDA as long as he didn't get upstaged by anyone on SALT matters. That may relieve Percy's concerns, though an aide to the senator said yesterday that he still hasn't had a chance to talk to Rostow about the matter.

Now there's the problem of filling in the cast. Michael Pillsbury, acting director of the agency, wants the deputy director's job, and reportedly has some influential backers, including Sen. Jesse Helms (R-N.C.), who backed Rowny for the top job.

But the expected choice is W. Scott Thompson, a Tufts University professor who acknowledges that he has been contacted about the job, but he's not sure the conversation constituted a job offer.

It is a little tough to tell sometimes. Rostow says both men are on his list of possible deputies. Beyond that, no comment.

If the choice is Thompson, he's likely to see a familiar face in the office -- that of his father-in-law, Paul H. Nitze, former deputy secretary of defense and, like Rostow, a member of the Committee on the Present Danger.

Nitze was once touted by conservatives for the No. 3 spot in the Pentagon and now is expected to be the head of the general advisory group at ACDA on what Rostow calls "my enthusiastic proposal."

So all is on line at ACDA? Well, maybe. But nary a single nomination has formally gone to the Senate, and the curtain isn't up yet.