Full House almost fired Ashley Olsen with the intention of only keeping Mary-Kate Olsen as Michelle. The Olsen twins got their start on the ABC family sitcom from the late '80s/early '90s playing the youngest Tanner sister. But at one point, only one of them was going to continue portraying the role.
One of the best things about watching Full House was how it chronicled the growth of every child character in its cast. When it first started, DJ and Stephanie were 11 and five, respectively. Meanwhile, Michelle was barely a year old when it went on the air. Unlike its spinoff sequel, Fuller House, the original series put most of its focus on the kids and their bonds with Danny, Jesse, and Joey, as well as their relationships with other people. Michelle, in particular, was integral to its overall appeal considering how reactive and animated she was, and that's mainly thanks to the Olsen twins' overall charm. Given how equally great they were in the role, it's difficult to imagine anyone else playing Michelle other than them.
There was a time, however, that Full House only wanted one of them to continue portraying the part. Initially, the pair looked very much alike, and one would have to resort to checking up on their dominant hand to tell if they were Mary-Kate or Ashley playing Michelle in a particular scene. But as the Olsens grew older, they became more distinguishable from each other. Given this and the idea that one Olsen would be able to carry the filming workload on her own, as Candace Cameron and Jodie Sweetin did in their earlier years, executive producers thought about getting rid of Ashley and only keeping Mary-Kate for the part of Michelle. Fortunately, before they could move forward with this decision, John Stamos stepped in and said no.
What's interesting about this scenario is that, early on in Full House, Stamos actually tried having the Olsen twins fired. This was when the girls were much younger, with the actor explaining that they cried too much, making it more difficult for them to film. They did try replacing Mary-Kate and Ashley then, but Stamos eventually realized they were still so much better than their other options, so he asked them to return. Considering how Jesse and Michelle became close in Full House, chances are Stamos eventually formed a personal bond with both girls. So when the idea of letting one of them go came about, he had already changed his mind about them. Hence, why he was adamant to keep both of them in the sitcom.
The Olsen twins continued to play the part until Full House ended after eight seasons. It's a shame they didn't return as Michelle in Fuller House, even though Stamos and the rest of the spinoff's cast and crew reached out to them multiple times. But as retired actresses, their decision to not revisit their old job and focus on being fashion designers was justified.