Will Smith and Robert Downey Jr. are only three years apart in age, and have spent some overlapping time as two of the most famous movie stars in the world. Yet their career navigations that got them to this point look vastly different. Smith logged a nearly unprecedented number of global hits between 1996 and 2008, largely predicated on his ingratiating persona popping into various genres (sci-fi, action, romantic comedy, drama). As it happens, 2008 was also the year that Iron Man turned Downey into a huge star after a couple of tumultuous decades in and out of the spotlight (not always for the best reasons).

Since then, Downey has appeared on screen primarily as Tony Stark, occasionally as Sherlock Holmes, and rarely in a non-blockbuster. Smith's career, existing largely outside the comforts of a clockwork franchise like the Marvel Cinematic Universe, has been more hit-and-miss, though he's still a big name. This weekend, they're both taking another crack at big star vehicles — albeit in the safer environs of mid-January, and possibly swapping strategies. Smith reawakens a long-dormant franchise with Bad Boys For Life (a three-quel opening nearly 17 years after its predecessor), while Downey drops his famous face into something new with the family adventure Dolittle.

Of course, in this context, "new" means a second remake of a widely reviled movie from the 1960s. Even so, Downey really is trying something in Dolittle, where he plays the famous doctor who can talk to animals (previously embodied by Rex Harrison and Eddie Murphy), surrounded by a bevy of computer-generated creatures with celebrity voices. Downey's own celebrity voice is weirdly and distractingly obscured by a low, mumbly tone and a Welsh accent, which immediately distinguishes the character from Stark or Holmes, and immediately brings to mind weirder, tic-heavier performances from Johnny Depp or Tom Hardy. The mish-mashy accent is especially Hardy-esque, though Downey finds less oddball music in his vocalizing, which thanks to certain camera angles and weird sound mixing often looks and sounds redubbed, whether it was or not.

The bigger issue is that much of Dolittle appears to have a litany of these clumsy post-production glitches, even by the slapdash standards of children's entertainment. The emotional journey Downey is supposed to be on gets telegraphed within the first five or ten minutes: Dolittle lost his adventuring wife, and has now shut himself off from the human world; will he learn to get close to people again? This means Downey once again plays a genius who alienates himself from humanity with his gifts. Here, instead of snark or a quick wit, the distance comes from a clumsily expressed melancholy — and by the movie's production issues. The story brings in a couple of plucky kids to draw Doctor Dolittle out of his shell, only Downey never connects with them as a performer, and the poor young actors spend a lot of time simply watching the story unfold as poorly rendered special effects clutter the frame. Downey never breaks out of that isolation; he's sapped of his sneaky energy and left flitting around in a vacuum, muttering through his shtick.

Will Smith also indulges in some movie-star alienation in Bad Boys For Life, as the story contrives to separate Miami cops Mike (Smith) and Marcus (Martin Lawrence) for a sizable chunk of its running time. Mike takes the movie's title seriously, stating his desire to run down criminals until he's 100, while new grandfather Marcus feels retirement might be in order. There's a lot more plot than that — a running joke about telenovelas turns out to be thematically appropriate — but as a star study, the movie boils down to Smith insisting he can keep doing his thing indefinitely, and Lawrence taking a more sensible view.

Bad Boys For Life isn't quite as insistent of its star's indestructibility as, say, a late-period Tom Cruise vehicle. Mike's defiance of his limitations doesn't scan as strictly honorable. But like a lot of star vehicles about aging, this one wants to acknowledge that process superficially without meaningful consequences. At one point, Mike is recovering from a serious injury, yet once he's out of the woods, he gets back to full movie-star strength with astonishing speed. And yes, Smith can still do his thing: he quips, runs, jumps, and summons intensity when needed. As ever, he's a true star.

Somewhere underneath his well-calibrated physicality, there seems to be some faint awareness that it can't last forever. This was closer to the surface of Smith's previous movie Gemini Man, where his super-assassin character fights off his own super-strong clone. A late-breaking plot turn in Bad Boys 3 creates some unexpected and bizarre parallels with that movie, while also chewing on way more than a 17-years-later Bad Boys follow-up can reasonably be expected to digest. The filmmaking in Gemini Man is better at teasing out a sense of regret, but neither Smith vehicle really follows through on its darkest impulses about what it means to age out of being the best in the game. Similarly, the basic material of Dolittle — a brilliant man coping with the loss of his beloved partner — may be close to Downey's heart (he collaborates closely with Susan, his wife and producing partner), which makes it all the more disappointing that it mostly just waits for grief to dissipate.

Bad Boys For Life and Dolittle aren't obligated to offer solutions to these problems. They're trying (and in Dolittle's case, noisily failing) to be multiplex entertainment, and even headier work can raise more questions than it answers. It's not a coincidence, though, that they're both most striking for the circumstances of their star's careers, and how they eventually arrive at such pat conclusions: Be close to people! Have relationships! Love is nice! Smith and Downey appear to have done this in their real lives, yet perhaps the need for reassurance that they exist outside of their famous personas still nags at them.

Bad Boys For Life has every opportunity to wrap one of Smith's few franchises, only to end promising more adventures to come. The Downey of Dolittle wants to embark on a new adventure, but has uncharacteristic difficulty expressing himself. Both stars are grappling with the isolation of superstardom — whether they realize it or not.

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