Announcements from talent agencies are always tersely non-informative. “Jim Carrey has left UTA and moved to CAA,” is about all that’s forthcoming in the typical communication.

When a star fires his agent, in reality, the circumstances are usually operatic. There is yelling and screaming. There are threats, real and implied, and there’s also hard-bargaining about commission splits and the like.

Behind the melodrama lurks a troublesome question: How do you measure good agenting? Further, why is it that the agent always gets the ax when a career hits a bad patch rather than the manager or attorney?

Consider these names: Jude Law, Tom Cruise, Mark Wahlberg, Orlando Bloom, Bruce Willis, Robert Downey Jr., Jake Gyllenhaal, Heath Ledger, Warren Beatty and Josh Lucas. Should their successes, or failures, be attributed to their representatives or to the performers themselves?

Jude Law’s performance was fine, as usual, in “All the King’s Men,” but his role was invisible. Indeed, he’s been invisible in film after film — remember “Alfie” and “Sky Captain and the World of Tomorrow”? — yet a couple of years ago everyone was calling him a star. Is he really picking his roles?

Orlando Bloom just opened in an obscure film called “Haven.” It’s smart for stars to take good parts in small films (think Travolta in “Pulp Fiction”) but Bloom is not just a late bloomer, he’s a non-bloomer. Does anyone remember him in “Elizabethtown” and “Kingdom of Heaven” or even in the “Pirates” sequels? (For the record, Law not long ago moved to Endeavor, and Bloom seems to be unrepresented in the States.)

Also missing in action: Remember the buzz Josh Lucas was getting a couple of years ago? That was before his “starring roles” turned out to be “Stealth” and “Poseidon.”

On the other hand, Mark Wahlberg has no pretensions of becoming the next Brando, but he keeps turning up in good roles in moneymaking films, most recently “Invincible,” “Four Brothers” and “The Italian Job,” not to mention “The Departed.” His production entity also gets enmeshed in TV shows like “Entourage.” Is the onetime icon of underwear that savvy or is he also being given some shrewd career management by Endeavor and his manager, Steve Levinson?

Then there’s Tom Cruise. His brain trust at CAA arguably made a superb deal at Paramount a few years ago — such a good deal that, when the final numbers came down, Cruise ended up way ahead of the studio. No one outside the star’s inner circle knows whether Cruise has been the engine behind this sort of killer dealmaking or his agents. In any case, Cruise has become an industry, not just an actor, and his career now must be re-ignited within a new kind of economic structure.

The agents for Heath Ledger and Jake Gyllenhaal (both at CAA) took a substantial risk putting their actors into “Brokeback Mountain” but it paid off big time. Their challenge now is to re-establish both in more traditional macho roles, and the jury is still out on this objective. Neither has appeared in a major hit in the two years since “Brokeback” went into production. (Gyllenhaal still had “Jarhead” in release during “Brokeback’s” run while Ledger’s “Casanova” came out three weeks after “Brokeback.”)

Also on the macho front, Bruce Willis, the ultimate action star, has found himself in five duds in a row after firing his agent and then his manager, and now is seeking to resurrect himself in the third sequel of “Die Hard.” (He has signed on with CAA.)

As for the famously self-protective Beatty, he has backed away from role after role as though in denial of the realities facing 60-plus actors. Robert De Niro managed to reinvent himself in comedy even as Clint Eastwood emerged as one of his generation’s great filmmakers. Beatty, however, hovers on the sidelines as a hardcore soccer dad while his cameos plugging the 25th anniversary of “Reds” serve as a reminder of his one-time prowess.

Which brings us back to the issue: Who does the steering and who is being steered? A star like Brad Pitt zealously guards his image and sifts his projects cautiously. The same for George Clooney, who intercuts “serious films” between his “Ocean’s Twelve” frivolities. Both Clooney and Pitt insist on being masters of their own fate.

By contrast, Brando in his later years told his agents, “Get me the fattest deal and I’ll do the part.” One top star once told me, “I always take the biggest offer because that shows the studio is behind the movie.” What he’s really saying is that he’s too lazy to read the scripts.

The problem stars face today is that there are too many voices competing for their attention — agents, managers, attorneys and random members of the posse. In a situation like this, the loudest voice wins.

So what should an actor expect from his agent? That he’s getting a shot at good parts; that he’s working with talented filmmakers; that when he’s down, his reps will rally behind him — witness a career as fragile as that of Robert Downey Jr., who’s gone from rehab to “Ironman.” (Downey is repped by CAA.)

There are some stars out there who apply a simpler measure: Is there anyone in my age bracket who’s getting a bigger payday? To be sure, it’s a perfectly reasonable criterion, but as the studios continue to toughen their negotiating stance, they, too, may have to reinvent their careers.

I’ve never worked as an agent, but I sense I’d be lousy at it. I’d tell my clients to chase the best movies, whatever the deal.

Of course I’d get the ax pretty fast.

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