Sonequa Martin-Green initially auditioned to play Michonne on The Walking Dead. One of the many ways that the TV show The Walking Dead tries to differ from the comics is by adding original characters. One of the show's most popular characters - Daryl Dixon - was created not only for the show but also specifically for actor Norman Reedus. In fact, when Reedus auditioned he read for Merle Dixon - in spite of the role already having been cast - and then Daryl was created around the actor's abilities. Another character created for the show and around the performer was Sasha - played for four season by Sonequa Martin-Green.
Sasha was introduced in season 3 along with her brother Tyreese, who does appear in the books. Over the last four seasons Sasha has survived longer than her brother and two love interests - Bob and Abraham - and struggled with emotional instability. She became reckless and hard for a time as she coped with losing Tyreese and Bob in quick succession - as well as her friend Beth. The former firefighter has also proven herself to be a hero and friend, aiding in numerous fights and taking care of those in need. And yet, the only reason Sasha even exists is because Martin-Green auditioned for another role and did not get it.
It's difficult to imagine anyone besides Gurira playing Michonne, or a show that never had Sasha in it. Certainly, if Martin-Green had won the part she auditioned for, things would probably have been quite different for Michonne on the series. Her story has deviated from the comics in a number of ways, and perhaps that happened because of Gurira's performance. Meanwhile, since Sasha was written specifically for Martin-Green, the writers were able to tailor the part to her abilities and skills.
Now Martin-Green is moving onto her next role, as the lead on Star Trek: Discovery. A chance she might not have been able to have if she played Michonne, who is still alive, well, and killing walkers with her swords. It seems as though not getting the role she auditioned for may have been the best thing to have happened to Martin-Green. Instead, she got to play an fantastic original character for years, and is now able to move onto her next great adventure.
The Walking Dead returns on AMC October 22, 2017.