Image taken by Joshua Waller with the 645Z
Creating a soft, out of focus background when you're shooting portraits will mean your subject will be the centre of attention and any distracting objects that are behind them are blurred out of view. If you want to create this look there are several ways to do it, without having to necessarily switch to manual, including a couple of ways Pentax and Ricoh compact camera users can create the effect too.
Using your camera's portrait mode is one way to get a blurred background, as the camera will then know that you want to use a smaller f/ number aperture (larger hole) to create a nicely blurred background for your subject. To make the effect more prominent, or to make the background more blurred and abstract, you can put more distance between yourself and your subject.
When using a Pentax DSLR, switching to a longer zoom with wider maximum apertures or a prime lens that's between 70mm and 150mm will help create a shallow depth of field, giving you the blurry background you're searching for. If you have a zoom lens use the longer end of the zoom rather than the wide as even though the wider end may have a larger maximum aperture, it's more likely to distort the features of your subject.
However, by using longer focal lengths, particularly with a wide to moderate aperture, will produce shots that flatter their features which is something everyone wants. Using longer lenses also means you don't have to work too close to your subject and as a result they'll be more comfortable and you'll have more natural looking portraits. Just keep an eye on your shutter speeds when using long lenses if you want to work hand held as you don't want shake creeping into your shots.
Using the aperture priority mode on your DSLR will enable you to have more control over the aperture you use, allowing you to experiment with the look different apertures give you, whilst still adjusting the shutter speed as necessary to make sure the shot is correctly exposed.