The Three Camera Method

Desi Arnaz knew that he needed the best he could get to help him revolutionize the way television was done.  For this, he chose legendary German cinematographer Karl Freund.  Together, they developed the three-camera method for filming the show, in which one camera got close-ups, one got medium shots, and the other got long shots.  Footage was then edited together during the editing process so audiences could see the show as a play, in sequence, and like a film, with different camera angles.  "I Love Lucy" was not only the first television show to be filmed, it was the first to use more than one camera, which became the standard method of filming sitcoms with live audiences ("Lucy" audiences were seated in bleachers across from the stage, with microphones hung above them.  The laughter you hear in every episode is from those original 1950s audiences, and never canned). 
During this pre-production period, Lucy and Desi started their own production company, cheifly because they wanted to have complete ownership of their filmed show.  CBS did not go for this at first, but agreed when Lucy and Desi compromised to a pay cut.  Their company became Desilu, with Desi as the President and Lucy as the Vice President (later, in the 1960s and beyond, Desilu was home to such shows as Star Trek, Mission:Impossible, The Andy Griffith Show, The Dick Van Dyke Show, and many others).
With the desicion made to film and own their show, the focus turned back to the cast.  Originally, Lucy wanted two actors that played another couple on her radio show, Gale Gordon and Bea Benaderet.  When neither could do the show because of previous commitments, the search began for what would become the other famous couple of "I Love Lucy": the Mertzes.