JOHN Cusack, always watchable and sometimes bringing a unusual take to a character (remember his cautious and sensible FBI man in the over-the-top Con Air?) is David Gordon in Martian Child. A widowed science fiction writer, he feels he understands what it is like not to fit in. He was, after all, a weird kid. So when he decides to adopt a child (as, apparently, his late wife wanted to), he seems a good match for Dennis, who lives under a big carton at a children’s home. Dennis, it seems, is afraid of the sun, seeing as he comes from Mars. It’s the logic of a child, not a scientist, but David meets him halfway, providing sunblock and sunglasses to coax him out. Martian Child is pretty sentimental stuff, but for me it didn’t seem horribly manipulative. It’s apparently based on a true story, and it certainly fits into the “troubled kid”genre.