Deft ‘Trainwreck’ keeps the laughs rolling

If you’ve seen “Spy” with Melissa McCarthy, you’re already aware that the movie nails its first big laugh — the sneezing-assassin joke — within moments of the opening credits. Even if you know it’s coming, the timing is just right. And right away you think: There. Thank you. These people know what they’re doing.

How often does that thought run through your mind in a mainstream commercial comedy? Not often enough. It didn’t happen with “Ted 2,” which may be a moderate box office success, but it’s a weak, vaguely smelly sequel.

I bring up these movies to give “Trainwreck,” written by and starring Amy Schumer, its full, brash, often riotous due.

At the risk of raising expectations, the first few scenes are among the best director Judd Apatow has ever done, in or out of the “40 Year-Old Virgin”/”Knocked Up” universe of arrested-development guydom. “Trainwreck” hails from the universe next door: arrested-development broadville. Schumer plays a fictionalized variation on herself, also named Amy, or more accurately a variation on the stand-up and “Inside Amy Schumer” Comedy Central personae that have carried Schumer to her current showbiz location.

The opening flashback sequence features Colin Quinn explaining to his two pre-teen daughters the futility and frustration of monogamy. Her childhood established in quick, deft expositional strokes — divorced parents; deceased mother; unrepentant horn-dog father afflicted with MS — we travel forward with Schumer’s Amy to the present. Her zesty, boozy, emotionally guarded love life includes more sex than love, but she doesn’t mind. Does she?

At any rate, she does not like her men to sleep over. Our guide to Manhattan romance writes for a sub-Cosmo magazine called “S’Nuff,” edited by a ferociously egocentric boss played by a barely recognizable Tilda Swinton. The plot of “Trainwreck” is simple. Amy’s assigned to profile a successful Manhattan sports medicine specialist, Aaron, played by Bill Hader. One abrupt but highly promising sleepover later, Aaron’s convinced they should date. Amy resists. The movie cooks up some conflict to divide these lovers for a while, around the two-thirds point, before Amy reckons with her more destructive and immature instincts.

The movie wouldn’t be much fun without them, of course. “Trainwreck” is all kinds of funny, and like any talent showcase worth its salt, the tone of the humor adjusts to suit the talents on screen. Early on there’s a bit in which Amy falls asleep immediately after a drunken orgasm, and it’s pricelessly managed. Aaron’s friend and confidant is LeBron James, portraying a penny-pinching, “Downton Abbey” version of himself. When these two have a heart-to-heart on the basketball court, it comes during the most lopsided game of one-on-one in history.

Apatow generally has trouble with his wrap-ups, and the final third or so of “Trainwreck” feels longish and full of detours.

The laughs in “Trainwreck” may come with an apology (the character describes herself as “broken”), but you believe the character’s transformation by romantic love, chiefly because Schumer and Hader are wonderful together.