There are no hard rules for this, as far as I'm aware, which is perfectly fine, as it's a very subjective label, but at what point would a person count as being familiar with another person for the purposes of inflicting a -5 penalty on the subject's Will save vs. a Scrying spell?
Without revealing too much of this scenario, in case someone from my game sees it, I propose several methods in which a person can become more familiar with another. I'm curious of others' interpretations in the methods' validity and time required to fulfill the stated parameters.
Cooperative Interaction - This involves speaking with the subject, fighting along-side them, eating with them, etc. It's getting to know someone the old-fashioned way.
This is the bar to measure by. Obviously, a caster Scrying on his father would count as being familiar, as would Scrying on his life-long friend. But what about a good friend he only met a year ago? I'd say yes, but how low does that bar go from there? Would you be considered familiar with someone you've known for a week - even after heavy interaction? I'm inclined to say no. I think the minimum here would be a month, and it would need to be regular interactions - the store clerk that you buy scroll paper from once a week for the past month would not count.
Subversive Interaction - This involves observing the subject in a way that the subject isn't aware of you - either by doing so up close albeit invisible or stealthed, or even using other forms of Divination, such as Clairaudience/Clairvoyance, or even in a situation where the subject is famous and their personality, preferences, and other characteristics are constantly on display for all to see.
Maybe some people wouldn't count this at all, but I think with enough time, it would be possible. Anyone here fans of long-running podcasts or Youtubers? I feel like I know Aron Hanson from Game Grumps pretty darn well at this point. For this, I personally would say a minimum of 1 year of dedicated observation would be needed.
The Diary - While reading about the subject through reports, research notes, etc. may not count for much, something else can be said about reading someone's personal diary/journal. With this method, the thing to measure would likely be the number of pages available, rather than the amount of time invested.
This is very similar to the Subversive Interaction, but in this case, you may not even know what the person looks like or what their voice sounds like. Still, a deaf caster should still be "familiar" with their father, even if they've never heard their voice. I feel like a blind caster casting Scrying is pretty asinine, but they would still be considered "familiar" with their father, regardless, right? Well, despite the visual and audible disconnect, there's still a case here imo, for the information revealed is likely extremely intimate. I'd even go so far as to say that the level of intimacy in the info you're getting balances out, letting you become familiar with the subject around the same length of time as Subversive Interaction; "Idk what they look like or sound like and I've never met them, but I know their deepest, darkest secrets, their preferences in fashion, their childhood dreams, their failures, their intolerance for chilly nights, and much, much more." So, assuming that one page is usually the space needed for a single entry, I'd say that around 350 pages would need to be read before becoming familiar with the subject (rounded down from a full year).
Detect Thoughts - This spell (and others similar to it) would be more of a supplement for either Cooperative Interaction or Subversive Interaction). Assuming the subject regularly fails its save vs. this spell, the caster would learn much more intimate details about the subject much faster than be external observation alone.
This is sort of a combination of either Cooperative or Subversive Interaction, and the Diary; in addition to all the info you're getting from your external observations, you're privy to the subject's intimate thoughts. I think a flat double-time modifier would be fair - using Detect Thoughts while traveling with someone for only 2 weeks would get you familiar with them, or using Detect Thoughts while spying on the subject would get you familiar in just 6 months.
Again, I know there are no hard rules for these, so I'm looking for opinions. Are there other categories of interaction I missed? Did I wildly overestimate how much time is required to become familiar with someone? Did I underestimate it?
The Diary definitely would fall under "second hand information" as far as scrying goes; to improve upon it you would have had to meet them (even if they don't know that they've met you); this is about as hard and fast as this gets; they have a table of increasing familiarity; secondhand is you've heard about them, the next step up is firsthand; you've met them. Familiar means that you know them well; to me that means you must have met them AND have knowledge of their habits and/or thoughts, so either of your interactions would work, detect thoughts would probably speed it up.
As for amount of time, it seems like second hand and first hand are basically instant; I think your time frames are a bit long, but, in that, it is totally GM discretion and you're the GM.
Of course, Rule 0 says the whole thing is GM discretion, so if it makes it more fun or interesting to allow the diary method, then do that.
I don't think there is any way to measure this out or apply a hard and fast rule. Depending on circumstances you could hardly know someone after years of acquaintance or become close friends with a deep understanding in a short time.
I think just observing someone could get you there, but it would be difficult. Certainly if you just observed someone in one context (say like listing to a podcast or watching a show) I don't think you would actually know them at all, all you are really seeing is a persona they are presenting, often after editing. Getting to know someone involves not only seeing them at their best, but also at their worst and everything in between.
I think a very personal diary that shared thoughts, hopes, dreams, fears etc. would certainly help get to know someone. Just reports on activities, others impressions and all of that would be secondhand, and wouldn't help much if at all. Similarly, detect thoughts doesn't reveal the deep nature of a person, just surface thoughts. Unless it was a nearly constant ability (not just a few minutes a day) I wouldn't consider it helpful.