For the first half of 1990, Fox will release movies that were already in the pipeline when Mr. Roth arrived at the studio. But he has spent an extra million dollars to shoot a new ending for ''Nuns on the Run,'' a comedy starring Eric Idle that will be released March 16. And he gave the producers of ''The Adventures of Ford Fairlane,'' an unorthodox thriller starring the comedian Andrew Dice Clay, an extra $1.5 million to make the movie bigger.

By June, 10 months after his arrival, bunches of Mr. Roth's movies will begin to reach theaters. It took two years for Jeffrey Katzenberg at Disney and Thomas Pollock at Universal to bring a full slate of movies to market. Fox intends to release 23 films in 1990, nearly double the 13 the studio released last year. More important, 13 of those movies will be produced by Fox, which only made six of the movies it released last year, including ''The Abyss,'' a $44 million underwater adventure that was waterlogged at the box office.

''You can't run a company based on one picture,'' says Mr. Roth. '' 'The Abyss' was two months late and 20 minutes too long. It's too difficult to make a great movie to have the fate of the studio rest on one picture. You can't sit around waiting for the perfect script. I came here and greenlit 15 pictures in three months. I didn't fire anybody. In 1989, we were in seventh place out of eight. I want to make this a first division team. For the second half of 1990, we'll be competitive with anyone.''

Sports metaphors dominate his conversation and his imagination. He turned himself into an athlete (baseball pitcher, basketball point guard, captain of the soccer team) ''to get away from the theological reverence of Communism in my house,'' he says. In each sport, he was the best defensive player - ''My job was to guard the best player on the other team or to feed the ball, a position that took single-mindedness and problem solving.''

One problem he is solving in an acquisitions meeting this morning is how much to pay for various movies that have been offered to Fox. ''Switch,'' the movie Blake Edwards is about to make, may be up for grabs. Fox has heard that female executives at Universal dislike the movie about a womanizer who is killed by three women and then reincarnated as a woman. In this era when theatrical release is only an appetizer, a main question is whether the videocassette of ''Switch'' will bring $6 million.

It is quickly decided that Fox will not bid for a new movie by Kenneth Branagh if the British actor and director of ''Henry V'' insists on final cut. It is equally quickly decided that the studio will try to talk Michael Douglas, Kathleen Turner and Danny De Vito into making a second sequel to ''Romancing the Stone.'' Because of the worldwide success of the first two movies, Tom Sherak, president of marketing and distribution, calls such a decision a ''no-brainer,'' even though the salaries of the three principals alone will come to more than $20 million.