For me, when it comes to PBs, I have to find the PB first before I can write the character, because their image has to be in my head to wind around. @_@
For Isobel... I knew I wanted blonde. I wanted a younger woman. I wanted undeniably cute. I actually ended up bouncing a lot of ideas off an old player here, Mika (Wounded Lion) before settling on Elisabeth Harnois because she was adorable.
As for Oliver... I had a hell of a time settling for him. I can't remember all the ones I looked at first, but Joseph Fiennes' eyes just kind of... pulled me in and I loved it. So. There we go.
Lucas was... fun to find. Initially, I had been looking at older actors, and then when we got the age range for them down, I narrowed it all in. Brendan Hines popped up and I loved all his expressions and that curly mop of hair and he just screamed FERNAND to me.
And then for Simon over at Pax... the guy I had initially settled on (Michael Gandolfi) was just too... sceney. Pretty. Too much guyliner and I couldn't cope with that. Then I found Ben Barnes and kind of drooled on the pretty and went through angsting because originally, Sakki had him on hold for... Oisin? I think. Then she changed her hold and I snatched him up and he's adorable and awesome and I can't stop staring at him. ;;