CHARLES BURNETT'S latest project is ``America Becoming,'' which he describes as a documentary about ``interaction,'' focusing on ``what the United States is becoming through the impact of immigrants'' and ``how established residents are changing their attitudes about what America is.'' When this is finished in the near future, Burnett plans another fiction film called ``Victim in a Mirror,'' about a detective who's too sensitive for his job. ``It's not really a detective story,'' Burnett says. ``It's about how if a prostitute is killed, or someone like that, nobody cares. People like that are treated like nonentities, like they were the problem. The movie is about how when a person really cares, he becomes the victim, in a sense.''