can i update datafields of a process instance without an action being taken

yes

in the process instance the is an update method

I agree with Gert, Just remember that the user execuing this Update method would require "Admin" rights on the process to do so.

If your case need to to have update via regular user then u can add action do so but flag this action as update action to stay at the same activity :)

you can have a look at my blackmarket project, http://www.k2underground.com/groups/k2_administrators_console_app_tool/default.aspx, there you can update a datafield from a command window or look at the sourcecode how i've done it.

I'll just add my 2c here. If you have a scenario where people NOT holding the Worklist item need to update the process's data frequently, you should consider externalizing the data using SmartObjects or your own tables. This makes modification of data simpler.

If you need to access the data in the process to do routing, set destinations, etc, it can still be done very easily using SmartObjects (or SmartObjects pointing to your custom tables).