Duet For One (1986, BLOCKED in the US) - Julie Andrews, Alan Bates, Max von Sydow

Duet for One By Janet Maslin (Feb. 13, 1987) IT took half a dozen extra characters and a very large tree to open up Tom Kempinski's two-character play ''Duet for One'' and bring it to the screen. But the focus is still Stephanie Anderson, a brilliant violinist (modeled, to some extent, on the cellist Jacqueline Du Pre) who is married to a celebrated conductor and has multiple sclerosis. Stephanie's illness becomes an occasion for her to re-examine her life as she describes it to Dr. Louis Feldman (Max von Sydow), a psychiatrist who is something of a friend and fan. It also becomes an occasion for Mr. Kempinski to elicit the audience's pity in a far too easy and automatic way. ''Why should I die such a horrible death?'' Stephanie asks in Andrei Konchalovsky's film ''Duet for One,'' which opens today at Cinema I. ''Was I so terrible? Was this music so terrible?'' A good deal of the screenplay, which is by Mr. Kempin