OVER AT WILLIAMS & CONNOLLY, the word is that Joe Califano, the half-million dollar lawyer, is coming back to the firm now that he's been fired as Secretary of Health, Education and Welfare.

Up at Cape Cod, where he's writing a book about his 30 months at HEW -- with a $100,000 advance from Simon and Schuster -- Califano says he hasn't made up his mind yet. But he's got some ideas.

If he came back to Washington to practice law, Califano says he's thought about also setting up a public-policy consulting firm to help states and cities and private interests organize their plans on issues such as housing, health and the environment.

He's also thought about politics, maybe running for the U.S. Senate from New York State.

And since Califano lost his job with the government, the young lawyers who joined him at HEW -- like Ben W. Heineman and Richard Cotton and Richard I. Beattie -- are trying harder than ever to persuade Califano to start up his own law firm. If Califano takes off on his own, presumably they go along in senior positions. If he returns to Williams & Connolly, their prospects change

Califano, of course, is well aware that Williams & Connolly wants him back.

"There's a lot of me in that law firm," Califano said in a telephone interview.

But, he added, "I have not decided what I'm going to do." He needs some time to think, he said, "It's been tough." So for the next several months, he's going to write his book.

"I want people to understand what this department is all about. What it's poentital for good is," Califano said. The book's working title is "Your Life and Mine" and it's set for publication in the fall of 1980, according to Simon and Schuster.

Califano the author says he will write about his personal experiences at HEW, which he described as "vivid enough" to make people want to read about them. But he will not disclose any best-selling details about his relationship with the White House.

"It's not going to be a Carter and Califano book," he said.

While Joe Califano writes -- and mulls over his options -- Edward Bennett Williams waits. He wants Califano back at the firm. And no doubt, Califano has not forgotten the rewards of a hugely successful Washington law practice. In 1976, the year before he left for HEW, Califano earned more than $550,000 as a senior partner at what was then Williams, Connolly & Califano. As one writer figured it, Califano made more money in a week than the average American worked earned in a year in 1975.

It's no secret that Williams has never liked the administrative details that come with running a big, powerful law firm. It's not his style. Califano has managed the firm before, and he could do it again.

There are those who say that Ed Williams would like to spend more time with his latest acquisition -- the Baltimore Orioles -- and with the Washington Redskins, of which he is part-owner and president. Williams says that's no so.

"I have plenty of time to give to my interests," Williams said in an interview.

But, he added, at the firm "I need all the heavy help I can get" and that means Califano's return.

"It's going to be great if Joe comes back and I'm hoping he will," Williams said.

Califano, however, insists he's not ready to be pinned down on a decision about practing law.

"We'll just have to wait and see," he said.

Whether it's fallout from President Carter's cabinet shakeup or just time for a change, government lawyers are in a shuffle. Various sources are sure that Deanne C. Siemer, now the general counsel at the Defense Department, will move to the Department of Energy with Secretary Charles Duncan, who was the No. 2 person at Defense. Siemer, who could not be reached for comment, repeatedly has denied any rumor of a change.

If Seimer moves, a possible candidate for her job is Togo D. West Jr., now special assistant to Defense Secretary Harold Brown. West is also considered a candidate for U.S. attorney in the District of Columbia.

Meanwhile, Bernhardt K. Wruble, the director of the Office of Government Ethics and formerly of the Defense Department, also has been helping Duncan with his transition to the Energy Department.

At the Justice Department, J. Michael Kelly, who was counselor to former Attorney General Griffin B. Bell, is expected to fill the same spot for Duncan at Energy.

Washington officials are accused of a lot of things, but the charge against Attorney General Benjamin R. Civiletti during a session with the news media last Friday night may have been a first.

"You're being deliberately uninteresting," complained National Public Radio reporter Nina Totenberg about Civiletti's response to a question about a judgeship for former Watergate special prosecutor Archibald Cox.

"I'm sorry," the attorney general responded.

U.S. District Court Judge Oren R. Lewis in Alexandria, whose caustic courtroom comments are at the legend stage, had a few words for his law clerk, Timothy Battle, who Lewis admitted to the Virginia bar last week.

"Now, I don't want you to get any misunderstanding," Lewis told Battle. "If you don't demean yourself in the manner in which you've been trained you'll be bluntly reminded of it."

"I wish you all the luck in the world," Lewis told his clerk, "Now get my (appointment) book will you?"

OBITER: Zona Hostetler, director of public service activities for the D.C. Bar, has been named to the American Bar Association committee on Ethics and Professional Responsibility. The committee issues the last word on interpretation of the ABA Code of Professional Responsibility -- rules of ethics for lawyers . . . Washington lawyer Ken Gozur has been elected to a two-year term on the State Committeemen of the Association of Trial Lawyers of America. The committeemen are the liaison between local trial lawyer associations and the ATLA Board of Governors . . . U.S. Magistrate Lawrence S. Margolis of Washington is the president-elect of the Judicial Administration Division of the American Bar Association . . . Harriett Taylor, formerly the city's administrative law judge, will be sworn in Friday as an associate judge of the D.C. Superior Court.