Setting values for custom display fields in Views
There are a couple of ways to go about this, if I understand your goals correctly:
1) If your view is set up to use Fields, you can customize the Subtitle field to include additional content along with what is coming from the Node's field.
To do this, Edit the view, Select the field, Expand "Rewrite results", and then you can use a Replacement Pattern token along with your additional text:  

The subtitle is [field_subtitle], yes it is!


2) If your view is set up to display as a Node, you would create a Node template override, named like node-contenttype_with_subtitle.tpl.php, and you can format your layout specifically for your need here:  

echo 'The subtitle is '.$node-&gt;content['subtitle']['#value'].', yes it is!';


Note that the 2nd approach would affect the appearance of that content type everywhere, not just in the view. So, if you want this special treatment of the subtitle only in your view, then the 1st approach may be preferable.

A third option would be a field-level override template, where you would create a file named something like views-view-field--subtitle--page.tpl.php and control your output there. Here is an in-depth look at that approach: https://drupal.org/node/352970

(Please let me know if I misunderstood your question, and I'll be glad to take another stab at answering.)