Should you choose your committee members or should your advisor? The question of how to choose PhD committee members has already been asked and answered in general terms, but I have the following more specific questions regarding the choice:


Is it more important that your advisor already know your committee members or that you do (e.g., your advisor knows them but you've never met them before vs. you know the member but your advisor doesn't have a strong or pre-existing relationship with them).
Is it better to get someone in your discipline or someone doing more related work (e.g., if you're getting a computer science degree, you ask computer science faculty vs. you're getting a computer science degree but everyone except for your advisor is from the English department)?
Should you pick people who are already invested/interested in you succeeding or is the dissertation process supposed to be a chance for you to win people over to your side?
You first have to check to see what the university rules are for a Ph.D committee. Many universities have rules that limit the number of external members, the number of members not from the department, the number of members that are not "regular faculty" and so on.

Assuming that you've checked all those constraints, then you should definitely discuss this with your advisor, who will have more experience in constructing committees in your area. 

Which brings up another issue: community norms. In your field of research, are there customary roles that committee members play ? that's something you need to discuss with your advisor. 

Finally, after all of the above, to answer some of your questions: 


  Should you already know them?


Not necessarily, although that helps with the first approach. It helps if they're familiar with your work even if you don't know them personally.


  Should they be doing research related to your dissertation topic?


Not necessarily, but they should have some connection to your work, otherwise they won't be able to provide any kind of useful feedback for you. 


  Should they be in your discipline?


Definitely, unless your topic is interdisciplinary and you want the input from someone in the other discipline. 


  Should they be selected more for how much they'll help you get a job?


That's definitely a factor. It's not a critical factor for all members of your committee, but it can be a factor when looking for an external member. Ideally, if you're able to do some research with members of the committee, they can write a letter for you. 

Bottom line:


talk to your advisor and discuss all of this with him/her
Different members of the committee can play different roles. The mix is what's important