How do you sound more casual in your voiceover? Lose control.
Are you holding on to sounding perfect? If so, try to let it go, it’s just words. Let them flow.
Are you using long-winter pitches that go up for a long period of time or down? Casual is short, it’s playful. It’s not flat, but it’s kind of like a deer trotting or a skier skiing down a mountain lose control and let it just flow.