I commented to Mike, "that tea is going to have something to do with it." He turned to me, surprised, and said, "Have you seen this before?"
No, I hadn't. It's just that Dr. House is a jerk and always treats patients like they're a pain in the ass. Suddenly jumping to offer the sister water so she could counter by pointing out that she'd brought tea was a red flag for me. Because Dr. House generally wouldn't do that. It was a giveaway.
The tea was, in fact, causing one of her symptoms, though it wasn't the cause of her main problem. Surprise.
This, in my opinion, is not the GOOD kind of clue, even in a mystery. I figured it out because the main character acted out of character. (And the show generally did a good job casting Dr. House as an insensitive jerk who cares way more about the thrill of solving the mystery than he does about the people involved. This was already clear after having watched only the first four episodes.) Clues to what's really going on shouldn't pop out to the audience because the character did something that was very odd for them to do.
Just yesterday, I was watching Gravity Falls (because people tell me I should), and I hit an episode called "Irrational Treasure." The main characters, Dipper and Mabel, were trying to find out who the real founder of their town was since they were trying to discredit the family of a girl who was mean to Mabel while on the run from the incompetent police who want to stop them, and throughout the episode she's walking around with a bag of butterscotch candies.
I don't know the Pines twins too well yet, but this was seven episodes in. I figured House out in four episodes well enough to know he wouldn't offer a nun some water out of the goodness of his heart. By this point, I had figured that both these kids are sweet and sometimes a little mischievous but aren't bad people. And then I saw Mabel eating one of her candies and carelessly tossing the wrapper behind her.
I said out loud, alone in my room, "Don't litter, Mabel! The people following you will find that!"
They did. The cops found the twins by following the trail of candy wrappers. It was plot-relevant.
It surprised me, because both of these shows are actually written very well most of the time. And it's not that big of a deal that I can figure things out based on small out-of-character clues; that doesn't ruin the shows for me. It just reminds me that they're written and constructed, and I don't like being reminded of that.
And then recently, in one of my new favorite shows, Steven Universe, I found a glitch that bothered me. In episode 32, "Fusion Cuisine," protagonist Steven hangs out with his friend Connie, and her parents are uneasy because they haven't met his parents. Connie lied to them and said Steven had a "nuclear family," which as she explains is two parents living with their children, and the plot's main conflict comes when they demand to meet his parents or she can't play over there anymore. This is a big problem for Steven because his mother "gave up her physical form" to make Steven, and though his dad is still around, he kind of has three moms--his mother's friends who watch over him. Now he's faced with deciding which one of them to bring to the dinner.
For most people who watch the show and know the characters, it seems like the really obviously most "normal" and "acceptable" "mom" would have been Pearl. She's overprotective (like Connie's parents) and she doesn't say or do too many inconvenient and difficult-to-explain things like the others. She's also not purple or seven feet tall. But Steven goes through the advantages and disadvantages for bringing each of them, and determines that Garnet's conversational skills are lacking and Amethyst is too "gross." (She picks her nose and is prone to making inappropriate jokes.) I guess the writers had to manufacture a reason for Pearl to have a disadvantage too, because the next part of the plot involves him finding a way to bring all of them, so they had her suddenly express an extreme aversion to eating. (These characters don't have to eat at all--they're aliens--but they have the ability to.) Not being able to make herself eat dinner at the dinner would have made Pearl seem really weird.
Now, I don't believe I've ever seen Pearl eat anything, so that's not a contradiction, and I have in fact seen the other two eat (well, Garnet drank something, while Amethyst eats all the time). And her reasoning is consistent with her character; she seems the most uncomfortable with physical existence and advocates calculated discipline. But there were other episodes where she interacted with food and didn't say or do anything that suggested it disgusted her. In one episode Steven chases his guardians around in an extradimensional temple because he's determined to have them all eat breakfast together, and at the end they are poised to have their meal and then agree to get pizza instead. She doesn't say anything about not wanting to participate. In another episode Steven even asks her if she likes pie and she exclaims "I DO like pie!" (Maybe she likes it for some reason other than to eat? Perhaps she just likes baking it but not eating it? That would actually fit her.)
It's true that in real life, people don't necessarily behave consistently. Real people sometimes act "out of character," though usually there would be some explanation if you were to investigate (regardless of whether the person themselves knows what it is). But the difference between real people and characters is that no audience is seeing the overall story arc of a real person's life and calculating how each out-of-character choice led to a thing that had to happen for the story to be entertaining to spectators.
As a writer whose story-building process is linked almost entirely to character, I don't decide what is going to happen and then have the characters move into their places and plug any holes. I construct the events of the story directly from what the characters would do. If I think of a plot that would require a character to act out of character, I have two choices: build up earlier events so the "out-of-character" actions will make sense in the larger context, or don't do it. And since all of the above-named television shows are written by multiple people and not all written at the same time, it's not always possible to do that, nor is it possible to retcon stuff that's already been created separately, greenlighted, or published/aired.
Sometimes you can even lampshade these things if you find them after the fact. Later episodes of House, M.D. could have shown the doctor reacting to others with an offer of water if they coughed and explained it as a character quirk associated with someone he'd known who had a chronic cough. Later episodes of Gravity Falls could have shown Mabel thinking candy wrappers are biodegradable or serving a positive purpose if released into the world. Later episodes of Steven Universe could have actively shown Pearl baking pies and having Steven ask her why the heck she bakes pies if she doesn't want to eat them. It might feel like stitching up holes in a suit that will never look new again, but it might also work to enrich the character. The best answer is always to not let plot necessities outrank established character traits, but once it's said and done, it can be salvageable.
Writers should keep characters' core traits in mind and double-check themselves when introducing an important new element. Otherwise, they'll end up reminding their audience that their people are imaginary and their stories aren't really happening to anyone, and that's at odds with why most of us enjoy fiction. There are of course exceptions, like absurdist humor and works where the fourth wall is broken, but that is an entirely different form of storytelling. In self-contained universes where we want to believe in the characters as real, consistency (or realistically portrayed inconsistency) is a must.