Thief developer Eidos Montreal has explained why it ditched the voice actor behind series star Garrett for the new game.

As revealed by the trailer released yesterday, Stephen Russell, who voiced Garrett in the previous Thief games, has not reprised his role.

Narrative director Steven Gallagher and audio director Jean-Christophe Verbert said in a community interview that while Russell was drafted in for preliminary voiceover work, the development team decided to go with a new actor for full performance capture.

"Stephen was considered quite heavily," Gallagher said. "We actually had him in for some preliminary vocal recordings in fact."

Verbert added: "Yes, it was actually very early during development at that time that we had him involved, before we made the decision to record our actor's voices and their movement at the same time using a full performance capture technique."

For Thief, due out next year for the PC, PlayStation 4 and next Xbox, Eidos Montreal is using a Naughty Dog-style full performance capture technique that means the developers are capturing voice, movement and facial animations all in one take. The actors wear a full mocap suit as well as a microphone and a helmet with a face-tracking camera during the process so EM can record the entire scene with every character all at one.

"Being able to capture the voice at the same time as the actors' movements and facial expressions, all while the actors play off each other, delivers a much more convincing experience than traditional techniques of recording each characters' dialogue separately and then animating everything by hand afterwards," Verbert continued.

Said Gallagher: "We want Thief to be a really rich, story-driven experience. Being able to deliver the narrative in the most convincing and believable way is a really important part of providing that experience."

So why did using full performance capture mean Eidos Montreal couldn't use Russell for Garrett?

"With the new visual of Garrett we'd created, we found that there was a disconnect that we couldn't ignore between the concepted character and Stephen's voice today," Gallagher explained.

"And to answer your question directly, the actor playing Garrett needed to be able to perform his own stunts," Verbert said.

"Garrett's a really athletic guy. We could have pasted Stephen's voice on top of the actions and stunts of someone else, but this wouldn't appear natural. It really wouldn't make any sense to capture the full performance for our other characters, but not for our star."

Gallagher concluded: "We're absolutely huge fans of Stephen's work on the classic Thief games. But when it came down to it, we ultimately realised that holding onto Steven as the voice of Garrett just for the sake of our nostalgia would be like wanting to cast Sean Connery again in the new 007 movies... It's a nice thought, but honestly, it wouldn't make sense for anything else other than nostalgia."