Even in a trifle, Natalie Portman is the real deal. If ‘No Strings Attached’ were built entirely around her character, we might be celebrating another triumph. Instead, we have to be content with a romantic comedy that is often quite funny, even as we’re constantly reminded that the two lead characters are hopelessly mismatched.

The fundamental incompatibility between Emma (Portman) and Adam (Ashton Kutcher) has nothing to do with their height difference — “When I stand next to him, people think he’s kidnapping me,” Emma tells a friend — nor with their differing career aspirations — Emma is beginning her career as a doctor and Adam is working as a production assistant on a TV show while waiting for his career as a writer to start.

Rather, it has more to do with the chasm between Portman and Kutcher as actors. Portman is completely believable as Emma, a relationship-shy woman with an incredibly busy, stressful job, who uses sex to release tension, provide pleasure, and avoid commitment. We know there’s must be a sizable amount of emotional baggage lurking beneath the surface; she’s too busy to dredge it up for the sake of a relationship. Some of that comes from the script, credited to Elizabeth Meriwether, and some comes from Portman’s performance. It’s a happy marriage of material and actor.

Kutcher, however, doesn’t add anything to the script, which paints Adam as a relentlessly positive person with “a good heart,” the latter almost a mantra, repeated twice by different characters. He’s pretty much a fantasy figure; his co-worker Lucy (Lake Bell), who has a crush on him, talks about his “distracting” good looks. He’s the consummate good boyfriend who has bad things happen to him: his father (Kevin Kline) begins dating his ex-girlfriend Vanessa (Ophelia Lovibond); despite making all the right, supportive moves, Emma keeps pushing him away.

Really, the movie needs to unfold entirely from Emma’s perspective, but because this is a Hollywood studio production and because stars need equal billing and equal screen time, we’re compelled to spend an inordinate amount of time with Adam. Which might be fine if his character had any depth; he doesn’t, because, remember, he’s a fantasy. And Kutcher is not able to convey anything else that’s going on with Adam. When he finds out that his dear old Dad, a former TV star, has been sleeping with Vanessa, he punches him, but does it in a nice way (it hurts him as much as it hurts his father). When he’s meant to look hurt and rejected, he just looks … like he hasn’t slept much lately. There’s almost nothing there.

Spending more time with Emma would also have allowed more screen time for her roommates: Shira (Mindy Kaling), Patrice (Greta Gerwig), and Guy (Guy Branum). And her mother (Talia Balsam) and sister (Olivia Thirlby). And experienced doctors on the job: Sam (Ben Lawson) and Metzner (Cary Elwes). All those characters have a moment (or two or three) to shine before rotating into the background. It’s a pity, because they’re all more interesting and amusing than Adam’s bland perfection.

Director Ivan Reitman, rebounding to some extent from the woeful ‘My Super Ex-Girlfriend’ back in 2006, keeps a loose handle on the reins. There’s plenty of merriment in the film, which plays out in a casual manner, to match the (initially) casual nature of Emma and Adam’s relationship. There are also clever, laugh out loud moments and, in spite of the consistently raunchy content in the dialogue, it feels basically inoffensive. It doesn’t really probe deep enough to provide a deep reaction.

With Portman on board, that’s a pity. ‘No Strings Attached’ could have been much more than it is, which is a pleasant, easily forgettable and formulaic romantic comedy.