The TV presenter and entertainer is due to appear in three episodes as cancer patient Tim O’Connor, the first of which airs next week.
The storyline will see Tim befriend ward sister Chrissie Williams, played by Tina Hobley, whilst receiving treatment for his cancer in hospital.
O’Grady explained to The Sun how took the role so seriously he had to tell himself he wasn’t really ill at the end of each day’s filming.
It was emotional filming Holby and I did actually come home feeling sick. I would lie in bed all day with tubes on me, and then I would go into Holby the next day, and they would put even more tubes on me.
They make you look ill, so after 14 hours of that every day, I came home and I really had to tell myself that I wasn’t actually ill as I wanted to do everything so much justice.
I literally had to tell myself to stop it, remember that I wasn’t ill and I was only playing a character.
O’Grady also says he had to convince a member of the public that he didn’t really have cancer after they overheard him discussing the storyline with his agent.
When I was first discussing it with my agent, we were walking down the street. I’ve got a voice like a foghorn and I asked her, ‘What kind of cancer have I got?’
She replied, ‘I don’t know’, but the woman in front overheard us and thought I was talking about myself.
I had to explain it was for a script in case it ended up on Twitter!
Although best known for his long career as an entertainment host and the star behind popular drag act Lily Savage, O’Grady has had previous acting roles with Eyes Down and The Bill among his credits.
O’Grady says he would like to pursue more acting roles in the future but would prefer a role in ITV period drama Downton Abbey over the network’s flagship soap opera Coronation Street.
They work such long hours in Corrie. I like the variety I have got now. But Downton Abbey would be good, yes.
I reckon I’d definitely be below the stairs cleaning out a grate or something! I’d hardly be upstairs, would I?
Holby City continues on Tuesday at 8.00pm on BBC One.