An LOI is usually worded to indicate that Actor X has read Script Y and finds it to be an attractive project from a content and commercial perspective. Actor X agrees to play Role Z in film Y based on financial, credit and additional considerations to be negotiated at a later date.
Before an actor or director signs a letter of intent his agent and/or manager vetts both the script and the people involved in the production. Consideration is given to the genre and the anticipated rating from the perspective of the impact it might have on the actor's image. Some actors will not do any R rated films at all.
After the rep oks the script as a desirable one for his client the client reads it and makes his own decision. There are cases where an actor doesn't bother with this step and relies on the rep's say so.
An LOI is not signed and issued casually both for the above described content/image reasons and because there is an industry perception value assigned to the number of projects to which an actor will express his intent in writing. The more LOIs an actor issues the less valuable and desirable he becomes to other projects. Additionally, an actor could find himself in a scheduling bind if he issues too many LOIs and speads himself too thin.
In Indy film making (as opposed to Studio) every contract is preceeded by an LOI. The contract serves to define compensation, percs, credit, etc issues. Any indy producer holding an LOI from Actor X is going to secure financing using the LOI as an enticement. So, even if the film does not get financed the actor is associated with it by his own choice. His name is being used to raise money for the project, whether it will be rated PG or NC-17. That's how the game is played and everyone involved knows the rules.
Thanks for the info but, Good Lord! that sounds like a complicated way to get a job! I'm making a face, too bad you can't see it. SAVED by the internet! LOL!!