In a BBC press release, she claimed that the Doctor Who star was "an inclusive actor".
"He'd just come back from Los Angeles and he was absolutely jet-lagged," she revealed. "They gave him this big scene to start with. He walked into this unfamiliar set-up and no matter who you are, I think you feel that."
Sladen promised that fans will enjoy Smith's appearance in the two-part 'Death of the Doctor'.
"He just had a week and he was really, really good," she said. "I think people will love it."
Smith himself recently claimed that he "loved" filming the guest spot.
The Sarah Jane Adventures returns to the CBBC Channel on October 11.