Michael Obiora, Charles Dale, Matt Bardock and Jane Hazlegrove, all former cast members on the ITV police drama, now play medical staff on the hospital series.
Of reuniting with his former colleagues, Walkinshaw told PA: "It did take the pressure off because it meant that if I did have a panic I knew I could shoot off, find someone and say, 'I don't know what I'm doing'. And they'd go, 'You're right, you don't', and then I'd be all right again."
Walkinshaw's popular role as The Bill's Smithy came to an end when the show was axed in 2009.
The 37-year-old turned up in BBC school drama Waterloo Road last year after Paul Nicholls unexpectedly left the part of PE teacher Jez Diamond.
Having defected to Casualty in March, he said of his new challenge: "It's a massive show, it's been around for ages and it's an institution.
"It's part of what our generation grew up watching, so it's important to treat it with the respect it deserves."