The more you underexpose and overdevelop the film, the more evident the grain will become. If you rate the film (Tri-X) at ISO 400 and develop for the suiting time it will be more grainy that if you rated it 320 or 250 (and develops for the suiting time).
Originally Posted by sterioma
Also, if you agitate too much it will lead to overdevelopment and, therefore, also to more grain.
Last I will comment on the use of Rodinal with Tri-X. This combo is GREAT, but it is a good thing to experiment with rating the film 250-320 instead of the 400 it says on the box. The reason is, that Rodinal decreases the speed a bit.
Please correct me if I'm wrong, but this what I have been told and experienced.