In Photo Booth, there are a couple of keys you can hold down to take pictures exactly how you'd like to. First of all, I'm sure you know that clicking the giant red camera button will take a picture.
if you hold down the Option key as you're pressing that button, though, it won't do the whole "three second countdown" thing before it snaps a picture—it'll just take it.
Yeah, I kind of hate the countdown, anyhow.
If you've chosen to take a video instead, the same thing'll happen; Photo Booth will start recording right as you click the button.
Secondly, if you hold down Shift while you click the camera button, it'll disable the screen flash temporarily. If you'd like to disable the flash permanently, you can do that, too. Just deselect Camera > Enable Screen Flash.
Finally, you can actually do both of these things at the same time. Hold down Option-Shift as you click the camera button, and you'll get a no-flash, no-countdown image. So you can see how weird you look at any moment of the day! In my case, that's often pretty weird.