No need to read this if you already know how it works, but in case you don't, I hope this helps to explain certain scenes.
Taking a bath
Unlike many western countries where it is normal to wash yourself in the bathtub, the actual bath in Japan is only used for soaking, while you wash yourself before you enter the tub. This way multiple people can use the same bathwater.
The bathroom itself is usually split up into two parts; a place to undress/ change your clothes/ fiddle with beauty products, and an inner bathroom where the shower and bath are located. This is the same for public baths, although on a whole different scale of course.
Once you've undressed and got yourself a towel you continue to the inner bathroom where you shower/wash yourself before entering the bath. Showering is usually done while sitting on a small stool. Might sound odd but it is actually quite handy. Usually there is a mirror in front of the shower as well. Most times the shower isn't left running. Instead, you shower briefly, wash your hair/body, and then use the shower again to wash away the soap. There is a drain in the floor for all the water.
When you are all clean you enter the bath tub. More fancy ones are heated so that you only have to refresh the water once in a while, or can even be programmed to fill the bath automatically at a certain time. Usually a bath is fairly hot, 40 to 42 degrees can be normal (and if you ask me that is scorching hot!!). You soak in the bath for a while, maybe take another short shower to cool down again, and then finally dry off and change into some fresh clothes.
If you google "Japanese bath" or "ofuro" you'll finds lots and lots of very fancy pictures, but something like this would be a more common bathroom in households. In any case there will almost certainly always be a bath, and never just a shower.
this ,or -with slightly more privacy- this. Onsen can have both inside and outside baths, which can be both natural or man made. The water from certain springs can be up to 90℃, so in some cases it need to be cooled before going into the bath. Both onsen and public baths can be either mixed or separate, with the latter being the standard (by law, if I'm not mistaken).
Wiki has lots more info on onsen and public baths if you're interested.
Back to Anata ga Ofuro de Noboseru CD...
In most of the Noboseru CD's the onsen spirit materializes in the water. They appear when you put the 湯の素 (yu no moto, somewhat like bath oil or bath salt) of a certain onsen in your bath water. Judging from the events in the CD's, putting that in the water is the first thing you do - before showering (but after undressing). Which seems logical, if you add something to your bathwater giving it some time to dissolve and mix would be sensible.
Either way, the onsen spirit appears and thanks to the guide not giving proper explanations you usually get a hell of a scare (sometimes hitting the poor spirit in the process).
When you've finally calmed down you return to the bathroom (or in case you are still there, get ready to shower). Most likely scenario there is that you are both sitting down on a stool, the onsen spirit probably behind you to wash your hair and/or back.
You enter the actual bath by the time the counting track begins. I have no idea why they included that, as you are obviously still in the bath after that in most of the CD's. In many of them the spirit eventually climbs in the tub with you, so for convenience sake we'll just assume you have a rather large bathtub.
CD specific explanations:
- In the freetalk of Vol.2 Morikawa-san talks about washing the mirror before washing himself. When you've got a bath full of hot steaming water or have just showered with very warm water the mirror in the bathroom is likely to get cloudy. Hence, him washing the mirror before anything else.
- In Vol.3 with Misasa Onsen he nearly drowns in the beginning when you put the lid on the bath. Most baths that have heating to keep them on a set temperature have some sort of cover. When you are finished you put the cover on the bath to minimize the loss of heat and save on energy.
In case of Diabolik Lovers...
For some reason I imagine the Sakamaki household bathroom to be unnecessary large. More like a small public bath than a normal bathroom really. I guess we'll just have to wait for the game to be released to see what I actually looks like. (I do hope they will include some bath scenes).
From Raito his CD it is clear that they have a changing room and an inner bathroom. When he puts you down in the bathroom you try to get away from him and run into the latter, and he leaves his own jacket in the changing room.
You've got enough space to struggle in there, so it has be decently spacey at the very least (but like I said, I imagine it being unnecessary large). In Subaru his CD you are in the bathtub together with him, so it has to be large enough for at least two people at as well. I'm not entirely sure whether you are both in the bathtub in Raito his CD as well. Things are a bit... vague in that one.
I hope this special helps to explain certain scenes better. If you have any questions after reading this feel free to ask them in the comments, I'll try to answer them to the best of my abilities. Likewise, if you feel I forgot something feel free to comment as well :)