The Mertzes

As in "My Favorite Husband", it was decided that Lucy and Ricky needed another couple to play off of.  In "Husband", the couple that fit that description were more financially successful than the Coopers (Lucy and Richard Denning), but after beginning to consider William Frawley, things began to change.  He was a seasoned character actor, having been in vaudeville for years and nearly 100 films to his credit.  Frawley personally called Lucy, whom he had known briefly when they were both under contract to RKO pictures in the late 1930s-early 1940s, and asked her if there was anything on her new show for him.  Lucy mentioned the call to Desi, who met with Frawley soon thereafter and immediately liked him for the role of Fred Mertz.  The more Desi championed Frawley for the role, the more he was met with opposition (namely because of Frawley's well-known alcohol problem), and the more Desi ultimately convinced producers Frawley was perfect for the part.  It was agreed that as long as Frawley was absolutely sober while working, they were willing to work with him.  Once he was cast, the role of Fred Mertz evolved from more financially successful than the Ricardos to less (mostly to fit his physicality and gruff persona). 
Once Frawley was cast, producers began the search for his wife, Ethel Mertz.  "I Love Lucy"'s first director, Marc Daniels, suggested an actress he had worked with on Broadway in the late 1940s, Vivian Vance.  Though she had had a successful career onstage, she was still a virtual unknown in Hollywood.  She was appearing at La Jolla Playhouse in a production of "The Voice of the Turtle" when Desi and Daniels went to see her (Lucy's first pregnancy was in too advanced of a state to travel at this point).  Desi absolutely loved her and she was hired backstage after the show.  Luckily, after just a few rehearsals, Vivian and Lucy bonded quickly, and would remain close personal and professional friends for the rest of their lives.