JOHN WILLIAMS CREATES THEMES FOR NBC NEWS

Tom Brokaw does not look much like E. T., but when he sat down to read the ''NBC Nightly News'' last night, the percussive buildup and gliding violins introducing him somehow recalled the long-necked reptilian in Steven Spielberg's movie. Somehow, the program put one in mind of - boys on flying bicycles. Spacemen. Sharks. Superman.

Why was that? Why did one see Mr. Brokaw but think of E. T.? The two have the same composer working for them: beginning last night, NBC News's musical themes will be supplied by John Williams, the Hollywood composer of soundtrack music for such films as ''Star Wars,'' ''Superman'' - and ''E. T.''

NBC commissioned Mr. Williams to write a series of themes for the network's news programs. ''We wanted to get away from synthesizer sounds forever,'' Tom Wolzien, NBC's vice president of editorial and production services, said, speaking of the recent vogue of electronic music on news programs.

Mr. Williams grappled with the situation and came up with the themes that NBC will run on its news programs from morning to evening. ''In the aural sense,'' said Mr. Williams, ''NBC has always been distinguished by the 'G-E-C' chimes. This will not replace that.''

'Statement That Is Accessible'

''They sent me cassettes of their news shows,'' he said. ''I watched some of the material but put it away. We wanted something that serves as an attention-getting melody, a statement that is accessible and hopefully not banal.'' He said he wanted to convey ''nobility of purpose,'' and ''consistency'' and ''the dignity that our networks can achieve and deserve.''

The network hired an 80-piece symphonic orchestra to record Mr. Williams's concept.

Mr. Wolzien said Mr. Williams described the components of his composition, which will run through the day on the network: ''There is the main theme, 'The Mission,' 'The Sound of the News,' which features a fanfare. Then there is 'Scherzo for ''Today,'' ' which Mr. Williams wrote after having a meeting with Steve Friedman, the producer of the 'Today' Show.

''Then there is 'the Pulse of Events,' which is more worldly and serious, with a sense of commitment and urgency, and the 'Fugue for the Changing Times,' a more celebratory and flashy piece.'' These might be used on documentaries.

Mr. Williams wrote 11 ''bumpers'' - the short pieces that feed into commercials and that are to be ''used as they fit for the news,'' said Mr. Wolzien. Mr. Williams described them as: ''Serious, contemplative, sprightly, light, breezy, vigorous, military, maestoso and gentle.''

Mr. Williams said he does not watch very much television. ''All the sound,'' he said, ''is flattened into a mezzo-forte.''