I also caught Michael McKean on Jimmy Fallon last night, and he said he found out about the part of Chuck from Bryan Cranston. They were both on the play All the Way on Broadway and McKean said a few minutes before they were to go out, Cranston told him he should do the "spinoff of Breaking Bad they are working on". McKean said "wait, what?" and then they had to go out on stage, and then after the show Cranston told him about the part of Saul's brother and that he(Cranston) had suggested McKean to Gilligan for the part and that McKean should expect a call in the next few days.