Writing is Like Theatre

I know I'm not the first person to come up with this, but I think it's true: writing is a lot like acting, and not just in the obvious ways.

I've done my share of acting—school plays, extracurricular drama … things—and I'd like to think I'm good at telling funny stories. I'm certainly a good mimic, if reactions can be trusted. I've also done time behind the scenes, in pit bands for school musicals. So while I've got no proper dramatic training, I'm familiar with what acting is, how you do it, and all the lovely fun that happens during rehearsals.

Writing is Like Theatre Because:

1. To be convincing, you have to channel characters. You have to know every little thing about them and bring that confidence forward, or the audience won't get sucked in.
2. It takes a lot of practice. You don't get cast as Hamlet if you've never acted before.
3. The audition process kills.
4. To act a part well, you've got to go over the lines until you've committed them to memory. To write well, you have to go over the manuscript until it's perfect, and you end up memorizing it in the process.
5. When writing fight scenes, it often helps to block out where all the characters are standing and moving to. When directing fight scenes, blocking is key, or nobody knows where to go.
6. There will always be critics. Not everyone will enjoy your performance.
7. Actors, like writers, specialize. It's rare to find someone equally good at Shakespeare, Hollywood action, and slapstick comedy.
8. There is always a moment (or fifty) during rehearsals when you seriously think the show won't go on. The lights break, the stagehands get confused, the lead actor gets a cold. But you go on anyway, and it turns out fine in the end.
9. Actors don't always do what they're supposed to. Neither do characters. Directors and writers must fight the urge to throttle them, and then work with what they're given. 
10. Actors, like writers, realize that they're all in it together, and tend to support each other, especially when they're working the same show/writing the same genre.
11. At some point during rehearsals, the actors will start to misbehave, play pranks, and goof off to relieve tension. At some point during revisions, writers are tempted to do the same.
12. Sometimes you have to go over a scene again and again and again….
13. The people on the sidelines (family, friends, stagehands, band members) don't always understand what it's like to be in the thick of things, or how the current project can absorb approximately 120% of your brain cells.
14. It can take weeks to build a set, find the props, and make the costumes. It takes a comparable time (minimum) to create and populate a world.
15. Actors and writers tend to display big egos while being on the verge of self-doubt a lot of the time.
16. You can tell when a production is amateur. It doesn't cohere as well.
17. Everyone says writers need to read their work out loud to check for bad writing.
18. The first 'novels' were oral.
19. When you get down to it, it's all about storytelling.

Of course, this list is based on my experiences, and my experiences are limited to amateur productions (often very amateur). I've probably missed stuff, and I've probably misrepresented the larger acting world through dint of not having seen it.* So feel free to chip in, add on, or correct!

* Probably the writing world too. I'm fairly new to it.