Whenever I watch a Cameron Diaz movie these days – such as Bad Teacher – I don't see the drama, I don't laugh at the comedy. Instead of a movie actress I once liked mildly for a season or two, I now only see an abstraction of the financial verities of modern movie superstardom. Her movies remind me of nothing so much as talent agency memos and aggressive star-packaging, multimillion-dollar pay-or-play deals, back-end points and foreign-market percentages.

She's in that top tier of stars among whom the (allegedly) choicest scripts circulate incestuously until one of them jumps ship or another climbs aboard. And it never really matters who ends up starring, with the result that they all – Roberts, Aniston, Jolie, Mendes, Diaz et al, plus their no-less-replaceable male equivalents – become essentially the same actor.

Take her last movie, for instance, the Cruiser-snoozer Knight And Day from odourless, flavourless hack James Mangold (I'd need two columns to convey the full strength of my loathing for that man's oeuvre). Diaz dropped in at the last minute to fill a role that had gone the rounds over the years, and had for a while been the property of Eva Mendes. Her male lead, Tom Cruise, his erstwhile leverage now much reduced after mid-decade idiocies and financial woes, parachuted in at the last moment accompanied by his own script-surgeon – having earlier failed to come to terms with the makers of the almost identical romantic thrillers Salt (his role was repurposed for Angelina Jolie) and The Tourist (losing out to Johnny Depp). Gerard Butler dropped out to make the witless and crude comedy The Bounty Hunter with Aniston, en route to The Ugly Truth (just ugly, not true) alongside potential Diaz-usurper Katherine Heigl. Even Adam Sandler was in the mix for a while before turning up in the blink-once-and-you'll-miss-it flop Just Go With It. With 10 writers involved at various times, Knight And Day ended up exactly as nutrient-free as the others I've mentioned. Why? Because they're all the same movie.

We hear a lot about the triple-threat actresses like Diaz face at this stage in their careers: time, gravity, and Hollywood's deeply ingrained ageism with regard to women on screen. The last is as real as the first two, but surely there is something no less deathly in the stage just before that, when one is finally, no matter the degree of one's success or renown, merely an addable/subtractable element in a movie's package, and susceptible to replacement by another star whose box office profile is the only thing they share with you. Roberts, Jolie, Aniston … no matter how many flops they star in, would all fit into the same space as Diaz, and no one would ever know she'd been there.

Once you get to this level, you're just a flavour or a topping, like cherry-vanilla, or pepperoni. Diaz's latest film won't change that.

Bad Teacher is out now