Shane (Jon Bernthal) and Dale (Jeffrey DeMunn) were both killed off in season two's closing episodes.
"You'll have to find out when you watch it - but there are certainly deaths coming out throughout the [third] season," Mazzara told Collider.
"And this is definitely a show where we take that stuff seriously. I think we showed that in our last few [season two] episodes. [Those] sort of hard-hitting emotional deaths are still going to be a part of the show for as long as it'll be around."
English actor David Morrissey will join the cast of The Walking Dead in season three as ruthless tyrant The Governor.
"[Series creator] Robert [Kirkman]'s got this great character named The Governor and we've cast David Morrissey to do that," said Mazzara. "Trust me - he is a lot to handle. Rick (Andrew Lincoln) and the group are going to have their hands full."
The Walking Dead returns to AMC in the US on October 14. In the UK, the series will air on FX later this year.
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Watch a trailer for season three of The Walking Dead below: