ON ACTING: The Courage to Feel
One answer is that certain scripts are written better than others--the age-old variant of: 'blame all performance difficulties on the writer and the dialogue.'
However, while I have found that the quality of the script is often an impediment to actors role understanding and execution, a more prevalent reason for actor problems is that the actor doesn't understand that part of his personality that is consistent with the character portrayal that is drawn by the script.
There is an old saying: "It takes one to know one." If an actor who is approaching a part does not have a keen and honest understanding of (and subsequently performance comfort with) the character/emotional side of themselves required by the script--for example, the killer side of themselves, the lover side of themselves, the sad side of themselves--they will have difficulty understanding and executing a character that is a killer, or a lover or very sad and weepy.
Actors are limited as much by their psychology as much as by their physiognomy. Actors are more often type-cast by their comfort-zone of feelings as much as by their body-types and height-weight-thickness.
If an actor wants to play a wide-ranging series of roles at the excellent level required by a top-notch successful career, they will have to demonstrate before a casting director a wide ranging understanding, acceptance and comfortableness with execution of wide ranging aspects of themselves.
They must (always consistent to script) understand, accept, and be willing to share before an audience the multiple side of their own emotional personalities. And realize that when they limit that range of understanding, acceptance and comfortableness in performance with his/her emotions, they are limiting the range of roles that they will asked/hired to perform.
The courage to feel--and the implications that that courage implies in the analysis, understanding and the willingness to reveal before an audience--is an essential (some would argue critical) aspect of an actor's craft and technique.