General Roommate Guidelines
(the minimum you'll need to know . . . or the minimum your roommates will WISH you knew)
Everyone is different in terms of what they can/not tolerate in another person. Living with roommates is a good way to figure that out, possibly as a form of "training wheels" for more important relationships. (Or as a refresher course if you just left one.)
If you can't learn to make it work with a more casual roommate relationship, you're pretty much doomed to mess it up when living with a long-term significant other-type partner . . . or doomed to look for new roommates an awful lot.
Be Specific about Your Desired Roommate Guidelines
A lot of folks looking for roommates in any roommate forum describe themselves as "laid back," "chill," or "easygoing," and looking for similar. However, your views on exactly what that consists of are not shared by as large a percentage of the population as you probably imagine.
Defined roommate guidelines lay the groundwork for a harmonious experience.
Vague seems like the cool way to be in the beginning, but in the end usually just leads to massive roommate-related misunderstandings.
Don't be surprised if an expectation is not specifically met if you didn't bother to specifically state it.
Now is Better Than Later
Best to go over these or any other specific household/roommate guidelines before the living situation starts, so expectations can be made known before anyone has to feel guilty or defensive afterwards for not having already done what they didn't know in advance they were expected to do.
But if ahead of time is no longer on the table, roommate guidelines as soon as possible are better than chaos.
Communicate with Your Roommate Using Your Nice Words
Do make your needs known, but do so as politely as possible. Don't assume that your roommate knows what your needs are unless you tell them.
Just because they seem to possess common sense in other areas doesn't mean they do with whatever you have in mind.
For many folks having a roommate is their first flail into an adult world without their Mommies and/or Daddies. For others it might be their first experience in quite a while sharing a roof with another adult after the breakup of a long-term significant-other relationship. Or, they might have had roommate(s) in the recent past, but folks that were different than yourself. They might have ways of doing things that seem as fine to them as yours do to you.
At any rate, if you don't tell them, assume they don't know. And assume if you don't tell them using Nice Words, they'll be less likely to want to comply with your needs.
How Dare My Roommate . . . %$#@ . . . ???
If you're angry with your roommate, verbalize why in as civil a fashion as you can manage, or just get over it, and quickly.
There's no other option . . . not an intelligent one anyway.
This doesn't mean be mean. It means don't be a doormat.
If you simmer yourself into a bitter emotional stew without communicating anything constructive, you have only yourself to blame.
Similarly, if you suspect your roommate is angry about something, ask them about it as politely as possible. But if they refuse to tell you and you really have no idea, refuse to feel guilty about it.
Know that it will be necessary, know it from Day One.
Yes, you want things your way. Who doesn't?
But having things your way all the time would probably mean having your own residence. Perhaps your own building. Maybe even your own block.
Reasonable people (or, shall we say, people that aren't asked to move on a regular basis) usually have to consider the needs of other humans at least part of the time.
Big Bad Words
Shouting, cursing, and just plain being mean won't get you anywhere either, because they're likely to escalate rather than resolve a situation. Sure, it might be nice to blow off steam, but behaving badly will just cause more "steam" to develop rather than resolving the situation so all roommates involved can get on with their lives already.
If there's a breach of what you feel were stated roommate guidelines, you may feel justified in continuing any hostility, but that still doesn't mean indulging in such will work out well for you, no matter whose fault it really is or is not. Continued screaming and/or passive aggressive behavior will negatively impact your life, no matter who started it or why.
If the situation is minor, resolve it by taking the high road. If the situation is major, resolve to find another roommate and get the hell out. But either way, resolve it, and do it soon. Don't be the next storyline on a tacky reality show!