Keepng important contacts on the home screen is handy. It help you to dail a call insatntly. It is a time consuming process to go in the Contact book and then Search for the person name and then tap and dial. I can give you a easy way to dial insatntly with one tap. For that you have to add the contact on the home screen. Xperia Z devices offer you a Top and Favorite contacts widget. While you can also do the same manually.
Step 1: Tap on the screen for sometime. You will get a menu at the bottom for customization. Tap on Widgets.
Step 2: From the list swipe to the right and tap on Favorites widget.
Step 3: You can swipe more to lcoate Recent Calls widget.
Step 4: While the last thing you can try is Top Contacts Widget.