I simply could not bring, myself to speak casually with them.

She spoke casually without looking at my mother, and changed the subject.

He spoke casually with just the right amount of interest.

Though she tried to speak casually, it came out between her teeth.

He spoke casually, which made it seem easier to start.

He spoke casually, intent on moving his glass to a new position.

He had spoken casually, but his eyes were intent on her face.

But I do think she's making an effort to speak correctly instead of speaking casually.

He spoke casually, as if telling them that the others were simply taking some routine time off and would return.

He had spoken casually, making no attempt to lower his voice; everyone was meant to hear.