Normally, the G12 focuses on whatever is in the middle of the screen. You can manually focus the lens before shooting by pressing the MF button and turning the Control dial.
However, once you begin recording, the focus is set: The camera doesn’t adjust the focus if your subject moves toward or away from the camera, which is one of the main drawbacks of shooting video with the G12 versus a dedicated camcorder.
It’s also possible to zoom in (but not out) on your subject while recording, but the zoom is digital, not optical, so the quality won’t be as good. It’s better to frame your subject before you press the shutter button.