For a time at the start of the Reagan administration, it was possible to think that Secretary of Defense Weinberger, being a budget man and having no special defense background, would concentrate on spending the Pentagon's old and new billions wisely and would tend to sidestep on policy issues until he had a bit more experience under his belt. But perhaps that was never in the cards, for something quite different has happened. Weinberger has given little public evidence of the beady attention to the budget that many of his friends and admirers expected of him, and he has zipped into policy matters that many of those same friends and admirers wish he had let slip for a while.

It is a bit awkward to say this because Caspar Weinberger is everyone's idea of a nice guy. If you are looking for villains in a Washington piece, there is Alexander Haig: a hands-on-hips strutter, someone who could scarcely conceal his certainty that he deserves to be No. 1 on the policy ladder, the man who was supposed to be doing Reagan a favor by taking charge of the world while the president tended to domestic details. Haig was a balloon waiting to be pricked, a political accident asking to happen.

But Cap Weinberger? Soft-spoken deferential in manner, someone who cocks his head toward you and listens while you speak, he wears his standing with Reagan, his successful career and his Washington experience lightly. He seems anything but the sort of Harvard man who can be told, but cannot be told much.

That just shows you, I suppose, how little we eastern provincials understood about California realities. Here it is just three months into the new administration. Haig, supposedly the strong man, has suffered a public, grievous and damaging fall from the charmed circle, if he was ever in it. Weinberger, the fellow whom some people thought might not be ready for the national security big time, is in the center ring.

This entails more than the pecking order. The policy-making process is not in balance. The proper order, in which heavyweight secretaries of state and defense offer their necessarily different personal and institutional perspectives to an open-minded president, has been disturbed.

It is not simply that Weinberger now regularly makes public statements on foreign policy, cutting in guilelessly, as though it were not trespassing on Haig's claim to be the president's foreign policy spokesman. It is that Weinberger's statements appear to be departmental statements, bearing a large measure of presidential approval, or indulgence, but not always reflecting evidence of having been worked out with the State Department.

The assertive anti-arms control statement on Poland that Weinberger coaxed out of the NATO defense ministers, going deep into the realm of substantive diplomacy, was an egregious example. The decision to add the AWACS planes to the F15 package for Saudi Arabia is another instance where, by various tellings, Weinberger came on strong with Pentagon arguments and rolled over the misgivings of diplomat Haig.

Haig, has been hurt by the infighting of past months, by some of his own actions and, not least, by White House reluctance to help him get his chief aides confirmed in the Senate. By contrast, Weinberger basks in presidential favor so complete it does not even have to be asserted. His policy staff is in place.

Right there is a delicate part of the difficulty. These aides are years deep into the policy arguments that Weinberger is joining only now. That Weinberger is a quick study is beside the point. I am not trying to hint that his aides, whom I know and respect, are leading him on. But I find it hard to imagine how Weinberger could have the full and subtle awareness of the policy recommendations being passed up to him that he would have if he had spent the last 10 or 20 or 30 years -- or nine months -- kicking those matters around.

Take, for example, the recent news story that Weinberger is instructing the services to plan for a longer conventional war than had previously been envisaged. His "guidance" does a lot more than rationalize a bigger budget, though that is no small consideration. It raises the fundamental and very abstruse question of whether war is best deterred by heightening conventional preparations, by threatening to go nuclear, or by improving the industrial's base for rearmament. Fred Ikle, Weinberger's undersecretary for policy, has moved in this area for years. Weinberger, the new boy, is off and running. The word at the White House is that he is merely working up options for later presidential decision, but I wonder.