Commitment to a cause can be an ennobling experience for an individual, especially someone who previously had had no commitment. Choices of the Heart (NBC, Monday, Dec. 5, 9-11 p.m.) dramatizes the life of Jean Donovan, a seemingly frivolous young woman who came to believe so deeply that she gave her life for a cause. She was the lay missionary raped and killed along with three nuns in El Salvador in December of 1980.
The crime has still not been completely solved - the guilty have not yet been punished. But that is not what this film is about. It is an attempt to look into the thought of Miss Donovan, the reason she gave up a life of easy pleasure to endure the daily hardships of the troubled ghettos of El Salvador. While it does not succeed, it is at least an interesting if slightly exploitative failure.
A couple of seasons back, the ''Crisis to Crisis'' series on PBS presented a documentary titled ''Roses in December,'' based on the life of Jean Donovan. Without melodramatics, it simply tried to plumb the depths of commitment of this party-loving college girl who developed into a mature and caring adult, deeply involved in improving the pitiful existence of impoverished Central Americans. It made no attempt to convert Jean's commitment into a facile psycho-drama.
''Roses'' did not try to solve the mystery of a developing conscience and social awareness; it simply told facts interspersed with authentic photos. The title was aptly derived from a letter Jean sent to her fiance concerning the indomitable spirit of the Salvadorean children: ''They know in December the roses will bloom again, and they believe this with a faith that is astounding. They are continually teaching me.''
Now, along comes the maudlin-titled ''Choices of the Heart,'' which tells the same story but endows it with facts and motivations that are supposed to have more dramatic impact. Jean is not merely a fun-loving coed, she is a promiscuous and basically vacuous child-adult whose dedication and commitment seem suddenly to flower full blown after a life devoid of serious objectives.
If there is faith involved, it seems to come out as a kind of greeting-card philosophy. Her conversion to a meaningful life of concern and commitment is hardly believable as portrayed by Melissa Gilbert, the Laura of TV's recent ''Little House on the Prairie'' series.
Where oh where is there convincing evidence of the subtle compassion, the irrepressible exuberance, which must have characterized Jean Donovan? Melissa brings earnestness and intelligence to the role but somehow misses the private grand passion which must have been concealed not far beneath the surface of the public persona of this ambivalent human being.
Written by John Pielmeier and directed by Joseph Sargent (who directed last week's superb ''Memorial Day'' on CBS), ''Choices of the Heart'' has its heart in the right place, if not its head. Good intentions are not enough . . . especially this time, when the film was preceded by a memorable documentary that evoked a more vivid image of Miss Donovan than this well-intentioned, manufactured, manicured, hoked-up version. Perhaps PBS can be prodded to rerun ''Roses in December'' soon so that the Jean Donovan mystery can be examined honestly once more.
A chat with Melissa
Anne Frank, Helen Keller, and now Jean Donovan. These were all unique, strong , and independent women. And in the past few years since ''Little House on the Prairie'' went off the air, Melissa Gilbert has played them all.
Does she identify with them?
In an interview, Melissa - pretty but not beautiful, bright but not brilliant , charming but not exactly charismatic in person - smiles, just a little embarrassed. She is the epitome of the girl next door, not necessarily on the prairie.
''Add Laura in 'Little House' to that list, too,'' she says. ''They were all strong women who found direction in their lives. Very strong willed. They followed their own instincts, which were usually right. Yes, I identify with them very much. I would like to have the memory of them with me always.''
Melissa, now a freshman at the University of Southern California, is very happy to be playing Miss Donovan. ''In most of the other roles I've done, I've been a teen-ager. Jean was a grown woman. It's a very important transitional part for me.''
She remembers ''Little House'' with great pleasure and hopes that, somehow, it is still not all over. There has been talk of several specials this year. ''I'll never forget the show,'' she says sincerely. ''It was 10 years of my life. I'll always remember the 'wrap' party when we all said goodbye.''