The 25-year-old, who is most noted for his role as Craig Dean in the Channel 4 teen soap, filmed his scenes as DJ Lenny earlier this week on a boat in Southampton.
Speaking of the part, he told DS: "It's an amazing character. I feature in a one-off episode where I play a late '60s radio DJ who's wired on drugs and booze - and he's got a gun.
"He's a completely over-the-top character. It's the best thing I've ever done - it's amazing. I've been filming all week on a boat."
The show's producer Ken Horn said: "We are so pleased to have Guy join the cast of the The Royal as a guest star in one of his first roles since leaving Hollyoaks.
"It's a really dramatic and exciting episode and Guy is perfect for the role of Lenny the DJ. We had a great time filming the scenes on a boat in Southampton and Guy was a delight to work with."
Burnet's scenes in The Royal are due to air in the early part of 2009.
Later this month, he returns to his role of Craig Dean for five episodes of Late Night Hollyoaks, scheduled to air on E4 from Monday, November 24.
Burnet quit his Hollyoaks role last August, but his character made a brief comeback in September for his much-anticipated Sunset Ending with John Paul McQueen (James Sutton).