Tobey Maguire.

HOLLYWOOD — Memo to Bradley Cooper, Sam Worthington, Zoe Saldana and anybody else who starred in a hit movie bound for sequel glory.

Don’t, to quote the mighty Beyonce, you ever for a second get to thinking you’re irreplaceable.

Just ask Megan Fox, who played opposite Shia LaBeouf in the first two Transformers movies, but not the third, currently in production.

Or Terrence Howard, who originated the role of Rhodey in Iron Man, only to see it commandeered by Don Cheadle for this summer’s smash sequel.

Or Twilight Saga regulars Kellan Lutz and Ashley Greene, who reportedly came this close to being replaced in the two Breaking Dawn movies over a salary dispute.

Undoubtedly wanting to avoid a Rachelle Lefevre situation (her Victoria character was assumed by Bryce Dallas Howard for the upcoming Eclipse instalment), Lutz and Greene ultimately opted not to push too hard.

The thing is, unless you happen to go by the name of Pattinson, Lautner or Stewart, these days there are very few instances where an actor is indispensable to the vehicle that shot him or her to the top of the box-office charts.

When it comes right down to it — sorry Shia — it’s those awesome Autobots and devious Decepticons that are the true draw of the Transformers movies.

Even Tobey Maguire and director Sam Raimi weren’t bullet-proof when the fourth round of negotiations started up for an addendum to the Spider-Man trilogy.

Contending that there wouldn’t be enough creative breathing room to meet Sony Pictures’ original May 6, 2011 release date, Raimi walked, Maguire followed and the studio gave the remaining players the boot and Spidey 4 a reboot, complete with a fresh-faced, younger cast and an origins storyline.

Even though Beverly Hills Chihuahua was a big fat hit, Disney decided that the main audience draw was those talking wiener dogs rather than the actors who lent them their voices.

So Beverly Hills Chihuahua 2 went before the cameras last fall minus the talents of Drew Barrymore, Andy Garcia, Piper Perabo and company.

In addition to new canine characters, Jamie Lee Curtis was deemed too pricey to reprise her live action Aunt Viv and was replaced by Susan Blakely.

Of course, it’s not always about money.

Sometimes actors don’t return for their encore as a result of those mysterious “creative differences,” which is usually code for “was difficult on set.”

In the case of Ms. Fox, the fact that she was declined a return invitation to the Transformers series could be attributed to various ongoing theories.

The first would concern her public disses of her two-time director Michael Bay, whom she has likened to both Hitler and Napoleon.

The second floating conjecture would have to do with Bay’s reported displeasure with Fox’s ultra-gaunt appearance, and since she wouldn’t agree to put back the pounds, he replaced her with decidedly curvier Victoria’s Secret model Rosie Huntington-Whiteley.

Then again, maybe Fox simply wanted a bigger paycheque.

Whatever the reason, may her Transformers 3 kiss-off serve as a warning to all those who think that a sequel gives them licence to play the diva.

To paraphrase Beyonce, they can have another you by tomorrow.

Michael Rechtshaffen, a Canadian entertainment writer based in Los Angeles, appears Wednesdays and Sundays.