APOLOGIA: Ah, Those Birthday Parties with the Family …

You know the joke that starts “How many (collective noun) does it take to change a light bulb?”

In APOLOGIA, the question that theatergoers are entitled to ask is “How much time does it take to change a light bulb?”

Not many more seconds than a minute, right? And yet, when Kristin sends out her grown son Peter to perform that task, he’s gone for an inordinate length of time.

Playwright Alexi Kaye Campbell needs him to. If he didn’t take his bulb-in-hand leave, Campbell wouldn’t be able to have Kristin and Peter’s new girlfriend Trudi engage in a private, serious, contentious – and long — conversation.

The two-act drama at the Laura Pels Theatre is wonderful in most instances but full of why-didn’t-the-playwright-think-of that moments, too. Think of a lobster bisque that you’re spooning into your mouth that’s so delicious – until you run into a tiny but annoying speck of shell.

They crop up much too often in APOLOGIA.

For example, Peter and his brother Simon are angry that Kristin, a noted art historian, didn’t mention either of them in her latest book. “It was all about the work,” she replies.

It’s a very good rebuttal, especially because the sons, at ages seven and five, were snatched away from her by their father. Given that the boys weren’t very much a part of her life – she was allowed to see them only the occasional holiday – why would the men expect to be cited?

They’d have a grievance if Kristin made no effort to get them back. In fact, did she? This issue isn’t much discussed – and it should be.

Campbell didn’t think so. She does imply that the reason the father took them away is that Kristin was far too centered on work. More needs to be explained on that situation, too.

Too bad these questions aren’t answered, for Campbell does otherwise offer smart writing in a familiar situation: a mother’s birthday party that sons feel obliged to attend.

(However, she could have told us if this is the first time or the umpteenth that they’ve acknowledged Kristin’s birthday. We don’t get a sense of that.)

Peter and Trudi are there to celebrate, as is Kristin’s old-time friend Hugh. Simon? He’ll “come later,” as his girlfriend Claire says after she arrives.

Kristin has always been somewhere between a liberal and a radical. (A framed photograph of Karl Marx is on display in her bathroom.) When Claire makes light of the anti-Vietnam War protests that she and Hugh joined, Kristin doesn’t come out with the expected and all-too-predictable invective. Campbell has Kristin counterattack by castigating Claire for not bringing Simon with her. As long as Kristin gets the chance to acidly express displeasure, that’ll suffice.

Later comes Kristin’s solid speech on the definition of an artist. It’s solely intended to trivialize Claire’s occupation – that is, if one can trivialize the already trivial: Claire is a soap opera star who has a hilariously euphemistic way of describing her daytime series. It show both a great lack of self-awareness and a need to be important.

Then, after something accidental and utterly disastrous happens at dinner, Campbell seems to be unaware of how people would genuinely react. For the next morning the person who caused the mishap and its victim act as if nothing of the kind had ever happened.

Even if Campbell didn’t write in the perpetrator’s remorse or the injured part’s hard feelings — which would be expected from both, especially only a few hours after the incident — director Daniel Aukin should have had his performers express these understandable and awaited emotions.

I’ve been seeing Stockard Channing perform since 1966. Back then at theaters at Harvard University and what was then Radcliffe College she did classic comedies (ARMS AND THE MAN), ancient dramas (EVERYMAN), theater of the absurd (THE LESSON) and new musicals (PEACE). I’ve never seen her give a performance that was less than impressive. That 52-year streak continues here.

Channing’s face can show heartbreak in many different ways and Campbell gives her many chances all night long which the actress seizes. Watch that face become steely when it needs to as well.

Campbell gives her a touching monologue about Giotto that also winds up challenging Trudi’s intense evangelical Christian beliefs. When she castigates Peter for his money-grubbing occupation — “For a living, you’re a taker, not a giver” – she does so in a matter-of-fact way that says that she knows he won’t disagree. Both of them know he can’t.

Although Campbell doesn’t always include necessary details, Channing does. After Kristin gets an atypical birthday gift, watch the way the actress discards the bag in which it was wrapped. If we had any previous doubts that she didn’t like it, now we really know for certain how she feels.

Because Peter and Simon don’t appear in the same scenes, Hugh Dancy is able to play both brothers. Here he gets the ultimate compliment that occurs when a performer does double-duty: you’d never know that the first actor you saw is the second actor, too.

As Trudi, Taline Monahon’s chirpy voice helps to underline that she has a great deal to learn and plenty of naiveté to shred. Her polar opposite is Claire. Megalyn Echikunwoke doesn’t just see to it that she extends false modesty when minimizing her accomplishments; she wants everyone to know that she’s putting on the modesty because that’s what celebrities are supposed to do while fully believing that they’re amazing human beings.

Noted director John Tillinger has decided to make a rare stage appearance. He’s Hugh, who has the familiar role of The Guy Who Comes out with Pithy Quips. Some defuse the situation and some add a ton of gasoline to the conflagration. Tillinger delivers each one so well that an audience will wish he had more of them.

And the title: Trudi makes the point that the word “apologia” is often used incorrectly. It’s not “apology” in Latin as many have inferred but instead means “a formal defense of one’s opinions or conduct.”

That’s a good detail. Would that Campbell had been as thorough with others. But for providing fully-drawn characters who know what to say and how to express what they absolutely need to, Campbell need offer no apologies.