Any time you write about an identified or identifiable person, you run the risk of being sued and, for a movie that has wide distribution, you open yourself up to not only the laws of Texas but of other states, as well.
In Texas, you would probably be safe from a misappropriation of a name or likeness invasion of privacy action, unless the individual you are writing about has a name that has some independent value to begin with (as would be the case with a celebrity). The Texas tort of misappropriation follows the Restatement (Second) of Torts which states, in essence, that it is an invasion of privacy to appropriate a name or likeness for the value associated with it. . . . . if the person can be identified from the publication and there is an advantage or benefit to the person using it.
To establish a misappropriation claim in Texas, there must be something unique and valuable about the person's name and it must be shown that you "cashed in" on the value of this name through your use of it.
If the person you are patterning your character after has a name or identity with no monetary value attached to it, you would probably be okay writing about him in Texas. However, the misappropriation of a name or likeness tort is broader in other states and can include ANY use of a person's name or likeness for a commercial purpose or monetary gain. You have the potential for being sued, therefore, in another state by this individual, under another state's laws.
Although Texas does not recognize the invasion of privacy tort of false light, other states do, and this, too, could raise issues for you with your script/movie. If your character is a less-than-savory person, for instance, this could place the real individual in a false light - people who know this real individual could assume he is as unsavory as your character is.
Your biggest worry, I believe, could be with defamation. If what you say about this identified or identifiable person, through characteristics or traits given the character in your script, is false about the real person, and it can be shown to have injured the real-life person's reputation (ie. your character is a drug dealer and the real person isn't), then you could be sued for defamation.
Your wisest course of action, whenever writing about a real and recognizable individual, is to obtain permission from the individual prior to writing about that individual and, certainly, before publication of any material about the person. Permission to use the name or likeness of an individual will often be required by any producer or publisher prior to purchase, production or publication anyway, as they will want to avoid any possibility of a lawsuit arising from the work.