Mitchell talked with ET Live on the site of the hotly anticipated Good Burger pop-up shop in Los Angeles, and he reflected on his happy memories of sharing the screen with Bynes.
"I love Amanda, she's like [my] little sister. She's so fearless," Mitchell, 40, shared. "I remember her being on set, she told me, 'Hey Kel, I wanna do the physical comedy that you do.' So I used to show her little tricks to do that would make her comedy just awesome."
"She did everything so great, so awesome, she was super fearless," he added.
While Bynes dealt with a rocky few years, she's rededicated herself to her health, fashion school and returning to acting, which Mitchell said seems like a great fit for the 33-year-old actress.
"She always had style, so going to fashion school was definitely what was up," he said, beaming.