I actually tend to have a vague idea of the character and then look for a PB so the faces and expressions can help me figure out like what exactly their personality is.
For Joanie, since she was going to be the Wicked Witch of the West I knew I wanted someone with a semi-large nose, preferably with a lump in it. So, and I know this is awful, I found a list of Jewish actresses on wikipedia and started going through them. After finding out that Amanda Bynes is Jewish, I clicked on "Kat Dennings" on the list. I looked her up and it was just instant. I was like "That's Joanie." I had never seen her in movies before, but I lucked out on her being so popular.
For Archer, I wanted someone with glasses. So I just kinda poked around a few different icon journals and found one that actually has nice symbols for things like "glasses" and stuff. Opened up a set of icons for JRM from this movie where he has dorky glasses, and I was like "Dude that's him." Again, pretty instant. Almost all his icons are from that movie because most JRM icons from other stuff look nothing like Archer in my mind, oddly enough.
For Brian, I actually had had the idea of making a Dr. Jekyll with Nicolas Cage for a while. I don't know how I came to have this idea, but I loved it. I love Nicolas Cage, I just think he's hilarious. So I had him, and needed a face for Max. I knew I wanted somebody with similar-ish features to Cage, like I didn't want to go from like Nicolas Cage to, say, Willem Dafoe. I posted on Find_a_PB and this person suggested Edward Norton, and it was just perfect.
For Gina, I started randomly thinking up actresses that I thought were maybe in their late 20's/early 30's and googling at will. Stumbled on SMG and was like, "...damnit that is Gina." The second I started hearing SMG's voice in my head saying Gina's lines, I knew it couldn't be anyone else.
And for Riley, I looked for people with weird hair. Found Hayley Williams. Loved her faces. The end.