Appropos of nothing, honest!
Staying in someone's house is not the same as living in your own place. The entire process is governed by a set of conventions and rules to try and cushion the inherent intrusiveness of bringing a new and temporary factor into a settled household.
As a guest, you should have two overriding goals. First, you must demonstrate in all of your actions that you know the difference between your host's household and a hotel. And second, you must strive to make a net contribution to the quality of life in the household. You may not succeed - it is very difficult to counterbalance the amount of work your stay imposes on your host - but a combination of helpfulness and good cheer will show that you are trying.
If anything, these rules apply even more when you are staying in a household run by family or friends than when you are visiting comparative strangers. The intimacy of your relationship makes your presence more emotionally complex, and it is important for continued harmony that you do as little damage as possible to that relationship.
If for some reason these customs are to onerous, one can always put up in a hotel, where there are no such expectations and where, as customer, you are always right.
Planning and Arrival
The process of being a houseguest begins long before you step through the door. When you are planning a trip that will involve staying at someone's house (either because you have been invited or because your relationship with your host means that you can ask to stay), ensure that you are welcome on the planned dates before you purchase any tickets. Do this yourself, even if someone else is booking the actual travel. Be sure to express your gratitude.
In the time leading up to your stay, keep in touch with your hosts. This will both ensure that they don't feel like hotel keepers and give you a continued thread of relationship to draw on for conversational purposes when you arrive. It will also allow you to update them on any changes to your plans, and accept or decline subsidiary invitations (parties, expeditions of pleasure) and requests for favours (babysitting).
If your transport is under your control, be timely, communicative, or both. Everyone understands that people set out later than they plan, but leaving your hosts awaiting you over a cooling meal is not a good way to start your visit.
If your transport is not under your control, do not automatically assume you will be collected from the station or airport. If your hosts don't offer to pick you up, plan another way to get to their house.
When you arrive, you will probably be shown around the house. Take note of the condition it is in - your hosts will probably have spent some time getting it clean and tidy. Your goal is to keep it that way, particularly the room you'll be staying in. A compliment on the decor or the house itself would not go amiss. Unless the guest room is actually crawling with cockroaches, assure your host that it's an ideal space and you look forward to being comfortable there.
At your earliest convenience (possibly after some conversation or food, if your hosts offer either), retire to your room and unpack. Find someplace to stow your possessions where they will be as out of the way as possible. By preference, things should not be behind doors, piled in untidy heaps, on the bed, or on the floor. This is particularly important if the room is multifunctional and will be used by your hosts during your stay. Remember at all times that this is your hosts' room, and that they may be feeling somewhat possessive and territorial despite their best efforts to the contrary.
If you have had the forethought to bring a small gift (often an edible one is suitable), bring it down when you've finished unpacking.
During your Stay
Ascertain all household rules and follow them. Areas to learn about include where food and drink are and are not allowed, whether shoes are allowed inside the house, where to smoke if you do so, toilet seat policies, swearing, doors open vs doors closed, and whether you have your own towel.
Eat what is put in front of you, compliment it, and be convincing.
It goes without saying that the guest room should be maintained in a state of tidiness.
Your goal as a guest is to have a light footprint, which means sharing the work of the household. Your hosts may very well put a lot of effort into making you comfortable, and may feel obliged to decline any offers of assistance you make. This can lead to some genteel argument. Remember to be neither a pest nor a parasite. It is often useful to find some periodic task and quietly take it over entirely - dishwashing is ideal in a household without a dishwasher.
Your hosts may lie to you while you are staying in their house. Common lies include, "You don't have to do that," "Oh, no, that wasn't in my way," and pretty much every usage of "That's all right." (Barring accidents, a handy rule of thumb is that anything worth apologising for is worth correcting, no matter what your hosts say to the contrary.) Smile and ignore their lies.
If your hosts feel it necessary to speak to you directly about something you are doing wrong, be aware that you have committed a major solecism and put them in an uncomfortable and unpleasant situation. (Any minor problem woul merely cause them to count the days till your departure.) Apologise and correct the problem immediately.
If your hosts have children and you are so inclined, you can offer to care for them. If you're not so inclined, don't worry. Most parents can distinguish between "kid people" and non-kid people, and if you're in the latter group, they won't be offended if you don't offer to look after the children.
It is often a good idea to take the burden of feeding the household off of your hosts at least once during your stay. This can either take the form of offering to cook a meal, bringing something in from a local takeaway, or taking them out to dinner.
It is important to contribute not just to the work of the household but to its ambience as well. Be pleasant and friendly to your hosts. If they enjoy a good discussion over dinner, feel free to express and argue your opinions against theirs then. Otherwise, if it isn't nice, don't say it. If you do stray into an argument and your host prefers not to continue it, agree to disagree and calmly turn the subject to something innocuous.
There may be brief times in a visit when a guest finds it difficult to maintain a cheerful demeanour. Fine judgement of the household mood is required here. If the hosts have the energy, and your relationship with them is sufficient, then of course you may rely on them for emotional support. Otherwise, the accepted practice is to retire to the guest room to express your feelings in private. The guest room is also a suitable retreat if the hosts have an argument with one another (although if the weather is good, a walk round the neighbourhood is better, requiring as it does no pretence of deafness).
Extended bad moods are best dealt with alone; better to be a quiet guest than drag the entire household into your depression.
Remember that, no matter how graceful and pleasant a guest you are, your presence will eventually tire your hosts. This is particularly true in a small house. It is a good idea to spend some part of every day out of the household, to give the hosts some breathing room. You could go on sightseeing or shopping expeditions, find a pleasant walk in the area, or adopt a local cafe. Smoking breaks, if they only lead to sitting just outside the house, are often insufficient.
When your visit draws to a close, express regret, even if you don't feel it. Your hosts will do the same.
On your last day, buy a small (usually consumable or temporary) present to express your gratitude. Flowers are a good choice, as is wine or food. If you give the gift to your hosts in person, you still owe them a thank you note. If you leave the gift in your room, you may leave a note with it.
Leave the guest room tidy. This will also reduce the chances you've left something behind.
If your hosts don't offer to get you to your onward transport, make arrangements well in advance.
When you arrive at your next destination, contact your hosts. You should both inform them of your safe arrival and thank them for their hospitality (even if you did so in writing when you left).
Never disclose private information you learned during your stay. Never even reveal that you learned it in the first place. In fact, forget that you read this paragraph.