Spartacus star Manu Bennett has been cast as the DC Comics character - the civilian identity of super-villain Deathstroke.
"It's nice to know who's playing Deathstroke, but it's not necessarily what you think," Amell told Give Me My Remote.
"[The writers] told me this idea [for Slade's introduction] they had maybe sometime around episode three. So for it to come to fruition now... I'm glad we had plans in the early part of the season and we're sticking to those plans."
Amell also confirmed that he had first met his new co-star Bennett when he auditioned to take over the title role in Spartacus in late 2010.
"I auditioned with Manu," the actor explained. "We shot a scene he had originally shot with [original Spartacus star] Andy Whitfield in season one."
He continued: "It's going to be intense the stuff that we do [in Arrow]... we do mix it up, but he's meeting me on the island and I'm far less capable. So mixing it up might be a nice way to put it. More like beating down."
Arrow airs on Wednesdays at 8/7c on The CW and continues tonight at 8pm on Sky1 in the UK.
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Watch a promo for the next episode of Arrow below: