[OS X TeX] Leading / Line-spacing Question

[Richard J Benish]

How does one change the leading of selected copy blocks without 
changing the leading of other copy blocks?

I've been trying to do this (without success) in a "letter" template. 
But I'd like to know (especially) if there is a general method that 
is independent of the kind of template (or document class) one starts 
with.

Thanks.

Richard Benish