On last Monday’s episode of “Louie,” comedian Louis C.K.’s show on FX, actress Sarah Baker, playing an overweight waitress named Vanessa, delivers a heartbreaking, mind-opening speech about the difficulties of finding love as a fat girl. This week, a lot of media attention has focused on the message (naturally, and deservedly so), as well as the fact that it was written, not by a fat girl, but by Louis C.K. himself (and kudos to him for his insight and tenderness). C.K. paints himself as the foil in the scene, the hapless adversary who sets off Vanessa’s impassioned soliloquy by asserting, in a ham-handed attempt at flattery, “You’re not fat.”

“Damn it. That is so goddamn disappointing, Louie,” she replies. “Louie, do you know what the meanest thing is you can say to a fat girl? ‘You’re not fat.’”So what was Louie supposed to say? The answer is probably: nothing.We’ve all been in the situation before. Your friend says something negative about herself, and you jump to contradict her with some blanket compliment like “You’re not fat!” or “You don’t have acne!” You both know it’s BS, but you feel like you have to say something.

That’s a natural reaction, says YouBeauty Self-Image Expert Heather Quinlan. But it might not be appropriate for every situation, and might be more about you than your friend.

“If it’s part of good-natured banter in the dressing room where everyone is taking turns at self-discouraging comments and compliments—not that that’s a good thing—but in that context it makes sense,” says Quinlan. Or imagine the classic confrontation of a woman coming downstairs and asking her husband, “Does this outfit make me look fat?” Quinlan laughs, “Obviously the right answer is never, ‘Yes, you look obese.’”

The interaction in “Louie” sets a very different scene. Vanessa isn’t fat-talking or fishing for compliments. “It was an actual serious conversation. And it was so obviously disingenuous what he said,” Quinlan remarks. The lie was far more insulting that the truth could ever be.You have to read the situation. Is it serious or in jest? How close am I to this person? How does the speaker feel about the attributes she’s describing? For example, a college student who comments that she’s not good at math probably doesn’t need you to swear that she is. “That might just be a ridiculous thing to say,” says Quinlan. “She might be accepting that this isn’t her subject.” It doesn’t need any response at all. On the other hand, if your friend is sitting on a dressing room bench sobbing with a bikini in her hand, that’s a good time to say something encouraging.

Here’s a rule of thumb to consider: If you’d have to interrupt the speaker to say what’s on your mind, don’t say it. “Louie didn’t like the half-second of silence,” Quinlan observes, and was compelled to interject to make himself less uncomfortable. “It didn’t have to do with her. He just wanted to make himself feel better.”It makes sense. It’s awkward to hear people put themselves down. And you don’t want your silence to come across as a tacit agreement that, yes, you also think your friend is fat, stupid, ugly, what have you. But it’s not about you. It’s. Not. About. You. What’s more, you owe it to the other person not to project your discomfort onto her. Just as the college student might not give a hoot that she’s hopeless at math, Vanessa wasn’t arguing that being fat is “bad,” and certainly didn’t need to be convinced that she isn’t heavy. “She was initiating it and she was comfortable with that characterization of herself,” says Quinlan. However foreign the concept might seem to many people, it is possible for a woman to self-identify as fat and not perceive that as negative.

If you just don’t know what to say in an uneasy situation, err on the side of not saying anything at all. In fact, that’s a lesson we could probably all stand to learn for life in general.

MORE: The 3-Step Body Confidence Makeover