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Like anything else, if it's critical to the plot, or if it would be very weird to leave it out, then put it in. If it's unnecessary or there's enough passage of time offscreen to cover it, leave it...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/19841 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/19841 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Like anything else, if it's critical to the plot, or if it would be very weird to leave it out, then put it in. If it's unnecessary or there's enough passage of time offscreen to cover it, leave it out. - If it's a hostage situation, everyone involved is going to be tense and focused for hours. The hostages may get desperate and sob that they have to pee, or the negotiator might be telling herself to ignore the urge because she has to stay on the line with the kidnappers. - If the character has eaten something foul or been given a poison, then the effects on the digestive system are important. There was a scene in one of the VC Andrews books, the Heaven series IIRC, where the main character is given a severe laxative while at a fancy party but the bullies lock all the bathrooms, so figuring out what to do and how not to ruin her outfit is a plot point. (She uses a dry cleaning bag.) - If you're describing the minutae of your character waking up and getting dressed, then using the bathroom is part of the morning ablutions. (as is _washing hands,_ don't forget that part) - On an episode of _House, MD,_ ["Top Secret,"](http://www.imdb.com/title/tt0979558/synopsis?ref_=ttpl_pl_syn) there was some weird side effect of his Vicodin use which prevented him from urinating, so he had to insert a catheter into himself. The viewer saw him on the loo, appropriately shot for broadcast TV, and then walking around with the catheter bag. A later nighttime accident (he's in bed and you see a dark splotch on the sheets) turns out to presage a typical House epiphany. In any case, tailor the description to what's appropriate. "He went to the bathroom." "He used the toilet." An old-fashioned phrasing is "She performed her morning toilet," which covers all the washing and eliminating. But if she's been poisoned, "her innards rolled and cramped. Horrible noises rumbled from her guts. She felt like she'd been stabbed. When at last she was able to push out the noisome mess, it burned and splattered. She felt like she needed two showers to get everything off." The reader knows _what_ is being described without actually, you know, getting the description.