'A MONTH by the Lake'' takes place on the shores of Lake Como, framed by exquisite Italian landscapes from beginning to end. This setting provides spirited competition for the film's performances, but the best of them shine brightly over their luminous surroundings. None is more dazzling than that of Vanessa Redgrave, clearly having a marvelous time in one of the most frothy pictures of her career. A review of Redgrave's screen credits - from ''Blow-Up'' and ''Julia'' to ''Howards End'' and ''Prick Up Your Ears,'' among many others - provides a quick reminder that she's always been more celebrated for serious drama than for effervescent romance. Her versatility is as strong as her talent, though, and in a recent interview she told me she thoroughly enjoyed her plunge into sunstruck comedy. Films with a comic bent ''have not been my speciality,'' she acknowledges. But she quickly adds that she considers herself ''the kind of actress who can respond totally to a situation. If the subject contains a good deal of joy and merriment ... you have a completely different mood than you do with another kind of subject. ''It's a common phenomenon for actors that something in which you can be light of heart, which makes you light of heart, and which you hope will make an audience light of heart, gives you a completely different feeling as you work.'' None of which means that filming ''A Month by the Lake'' was more a carefree vacation than a professional challenge. ''It's just as hard to make something that is light of heart,'' Redgrave says, echoing many performers who have named comedy as one of the most difficult acting chores. ''Your moods are affected, but just as much work goes into it.'' Redgrave has kept up a busy cinematic schedule lately, appearing in movies as different as the mournful ''Ballad of the Sad Cafe,'' the colorful ''House of the Spirits,'' and the gritty ''Little Odessa'' in recent times. But she also remains active in stage work, as an actress and a key figure behind the Moving Theater, a British acting company that hopes to make its American debut next year. ''I think it's terribly important for actors to be able to do both film and plays,'' she says. Redgrave laments the fact that current political and economic trends are making this more difficult by cutting the resources of stage companies even in her native England, which has a stronger tradition of subsidized theater than the United States does. ''I think I'm the only person I've ever heard say such a thing,'' she continues, ''but I feel very strongly that a major source of my development as a theater actress has come from work in cinema, and not the other way round.'' Lessons of the cinema ''Film requires living in the moment and creating in the moment, whatever that moment is - and doing it out of chronology, since films are made in the editing, not in the linear sequence of a classical play.... For all of life's contradictions, the cinema is 'ace' at showing it as it really happens.'' Redgrave also puts a high value on the human interactions that are integral to good ensemble work. ''Every day you're working with actors who are one day older,'' she says. ''Your director is one day older, and you yourself are one day older. You've changed, however unapparent that might be. Since your starting point has to be who you're with, not 'me and my part,' there's a synthesis with the script, which has a life of its own anyway. Then the play and characters begin to emerge of their own accord, as it were - to have a life bigger than all the thoughts of the director and the actors combined.'' Redgrave takes an unusual position on the much-debated question of which acting style gives better results - the ''Method'' approach of drawing on one's internal emotions, or the ''external'' technique based on outward appearances. ''They are two different approaches,'' the actress says, ''but they can arrive at the same place. Some actors distinctly work one way or the other, and some would die rather than actually feel what they act feeling. ''I work dialectally, from out to in and from in to out, at the same time.'' In a similar vein, Redgrave prefers not to choose between ''intellectual'' and ''intuitive'' ways of preparing a role. ''I tend to put in a lot of intellectual work,'' she says, ''although sometimes I don't need to, or don't think I need to.'' To illustrate her thinking on this subject, she cites an early stage role that she approached on a ''purely intuitive'' level. Even though she acted ''without thinking it through,'' she spent a large amount of time writing diaries that reflected the thoughts and experiences of her character ''so I could see in my mind's eye all [the character's] life, not just the particular scenes that were in the play. This didn't change any of my actions or impulses, but it nevertheless deepened the work.'' Politically outspoken In past years, Redgrave has sometimes made more news for her fearlessly voiced political opinions than for her purely professional activities. While the press has lately paid less attention to this facet of her life, she remains heartily concerned with world events and shows a detailed knowledge of current situations - most notably the war in former Yugoslavia - that trouble her. She's as outspoken as ever, she insists, even if journalists have eased off their coverage of what she says. Still, it's clear that the core of her life remains her work as an actress. ''Having got to the age I am,'' she says with a smile, ''I still absolutely love the theater, I love cinema, and I love the work of performing, investigating, developing productions. I haven't lost any of my enthusiasm whatsoever. In fact, I've probably got more than I ever had.''