Sam Waterston, appearing in ''MindWalk,'' currently playing at the Senator Theatre, says he became involved in the project because of Liv Ullmann, who was already cast as a physicist. It was she who recommended that Waterston be asked to play the American senator who visits Mont St. Michel where he, the physicist and an American writer discuss, at length, the state of the world, more specifically, the ruination of the planet.
''It was my good deed in a wicked world,'' said Waterston.
He was calling from Washington, where he had attended a screening of the film. ''I knew the material was based on the writings of a very smart man, Fritjof Capra, who makes esoteric material accessible to us mortals,'' he said. ''It has to do with the preservation and salvation of the earth itself.''
It was suggested that the film doesn't tell you much you don't already know. Waterston said he didn't agree with that and that even if he did, it wouldn't hurt if a lot of people were to hear what the film has to say.
He said he particularly wanted to do the movie because, ''I thought it was impossible on an artistic level. It is the talkiest film I have ever seen, but I always wanted to do a talky movie, and this is the ultimate. If 'My Dinner with Andre' worked, then this one can, too.''
He says he has no commercial interest in the movie. ''But the producers do, and I certainly do wish them well,'' he said.
''I'm doing a television series, 'I'll Fly Away,' and there were those who said it wouldn't work, that anything with substance, as opposed to fire and death, wouldn't work.
''I am told that 'MindWalk' is performing nicely,'' he said. ''It wasn't meant to compete with 'Terminator 2.' ''
He enjoyed doing the film. ''When you do a good deed, you act out of passion, so the atmosphere was more charged,'' he said.
Waterston has done some good and bad films, most of which have made it to television. He's not especially pleased with that. ''The trouble with television is that nothing ever dies,'' he said. ''Your best films keep on showing, but so do your worst.''
He wouldn't say which he would consider his worst. ''I'm not going to go down the list with you.''
''The Story of Boys and Girls,'' opening today at the Charles, is a small, Italian-made film that seems to do nothing for 45 minutes or so, then becomes amusing.
Its point is that people are the same the world over, and when you bring two large families together, there will be flirtations, clashes, small delights and large embarrassments.
The film takes place in 1936, at the countryside home of a farm family whose members are awaiting the family of the boy who has proposed marriage to a member of the host group. The visiting family is from Bologna and considers itself above the farmers.
The engagement dinner is an elaborate one. The host family spends hours preparing for it. The guest family is not sure it wants to partake, but it does.
The girl's father is a roue who cheats on his wife. He is remorseful only when he is informed that he was not the first man to make love to his new mistress.
Another member of the host family comes on to a member of the guest family. Actually, there is quite a bit of this going on, which is what the film is all about, the things that happen when two families meet and members of one make fools of themselves.
''The Story of Boys and Girls'' will remain at the Charles through next Wednesday. Give it some time, and it is enjoyable.
Add the name of Paul Mantee to the names of those actors who are also authors.
Mantee, who may be best known for doing the lead role in the 1964 ''Robinson Crusoe on Mars'' (he also appeared, regularly on ''Cagney and Lacey'' on TV), has written a book called ''In Search of the Perfect Ravioli,'' the story of a newly divorced screenwriter suffering writer's block who turns to his pasta machine for inspiration.
Have a screenplay you'd like to sell? Don't know how to go about it?
Well, now available at your local video store is a cassette called ''Writing and Selling Your First Screenplay'' by Glenn Benest.
According to the cassette sleeve, Benest is ''a leading screenwriter and lecturer at UCLA and the American Film Institute.'' The tape runs approximately 90 minutes.