My standard setting for the autofocus on my Rebel is ‘Single’. Simply because I want to be able to lock the focus even in the case that I move the camera slightly.
Especially if you take a portrait of one of our friends and you set your focus on ‘Continuous’ not only the mode will drive you nuts because of the never ending lens movements, but also the chance to get a portrait that isn’t sharp at all will be bigger.
So in this case your choice for this purpose would create the opposite of what you wanted.
When it comes to fast action like, let’s say, owls taking off of a tree trunk you should choose the continuous autofocus to have a bigger chance to get a proper and sharp image.
Because I kept my focus on single mode the following images are sharp by accident. You have to take into consideration that with the shutter speed used I really hadn’t the time to lock the focus deliberately.
If you click on the images you’ll see a larger version. Under the images I put the settings for your information.