Wait no more for I love questions!
How do you choose what you're going to ask someone? Is it just random or are there other factors that come in to play?
Also, when is it the best time to focus in on one person? For example, when you have a gut feeling/your a cop and know they're scum or when you have a fair amount of "evidence" against them? Or I guess what I'm really asking is IS there a best time?
Oooh, ohh! I'm maybe
the a future IC someday, so I'll offer a perspective too:
There isn't really a 'best time' for questions or narrow or broad focus, but timing can matter, it's situational.
Never forget your goal. If you're Town, you want to find the Scum, and everything you do in play is ideally going to somehow help you find them, from the questions you ask to the timing you use to the specific wording you pick - or not. Depends on you, and if that's not your style, then you'll need to find and develop your own.
If you're Scum, then you need to appear to be Town and appear to want to find the Scum, but you need to do so in a way that helps ensure that the fewest possible Scum are voted for - note that may mean scumhunting your Scumbuddy and helping Town lynch that player, even on Day 1. That's called bussing - and is not an ideal tactic - unless in your exact circumstances it is the ideal tactic.
As to what questions to pick to ask who - I recommend you read over a few of the finished BM games, maybe just read the first few pages after start of play for now from a handful. See what others asked as their opening questions or whatever else people started with, see how well or poorly that seemed to work. When I played my first BM I'd not yet read any games. As I started to play, I started to read 3 previous BMs, only to the approximate point I was at in the one I was playing. As the game progressed, I read a little more. When things seemed harder or weird, I started looking at other finished games, non-BM games, and checked out what had happened in those games around the point mine was at.
General question advice is 'ask questions related to Mafia in some way', 'follow up on the answers if you see anything to follow up on', and 'ask both the types of questions you don't know the answer to, and the sort of questions that you think you -do- know the answer to'. Like what did someone mean when they said something (quote it when you ask). Ask someone what they think something means that someone else answered or asked.
As you talk to people, you're likely to notice things that make more or less sense to you, see things that seem off or weird. Ask people about them. Give people chances to explain themselves (that was perhaps my biggest fault in my first game, I -leapt- to lynch people I thought were Scummy. Those people -were- Scummy! But they were not actually Scum. More communication might have helped straighten that out and avoid a mislynch). Question, question more, keep questioning. When things make sense and feel right, maybe they are. When things seem off and wrong and you're finding inconsistencies, maybe they are.
Consider paying attention to what everyone is doing as best as you can. You might find very interesting things in the questions and answers that are not directed at you, and you can and potentially should ask about that interaction as well.
One potentially cautionary note - you may (or may not) want to consider not asking about someone else's question until after it's been answered (or some time has passed and it's clear that it won't be answered anytime soon). There's more than one school of thought on that one, but some people feel that 'jumping in' like that changes how the question's target is likely to answer, and may take pressure off of Scum (Scum may have been thinking 'OMG I got asked a really hard to answer question, what do I do!' then see your question and think 'Oh, that's just a really weird question. Thank goodness that guy got involved, now do I want to attack that guy for jumping in or attack the guy that asked me the question for asking such a weird question?' or something along those lines). Others feel there's little or nothing wrong with asking about what someone else asks immediately - the interaction as a whole, what gets reacted to, and what appears not to be reacted to, tells a useful story and 'jumping in' has a place and a value, at least sometimes.