John Simm's return as The Master in 'Doctor Who' is set to be a major part of the plot of the new series.
The actor is making his return to the BBC One sci-fi show after first portraying the evil Time Lord in 2007 in the final three episodes of Series 3 - 'Utopia', 'The Sound of Drums' and 'Last of the Time Lords' - when he battled Tenth Doctor David Tennant.
Simm then came back at the end of 2009 for Tennant's farewell story 'The End of Time' making his final appearance in the role on New Year's Day in 2010 as Tennant regenerated into Eleventh Doctor Matt Smith.
It was announced last week that Simm would be reprising his role as The Master opposite the next regeneration of the time traveller Missy - played by Michelle Gomez, the first woman to portray the villain - and Twelfth Doctor Peter Capaldi in his final series.
'Doctor Who' executive producer Brian Minchin has now teased that viewers will be seeing a lot more of Simm than they perhaps initially thought as he spent weeks filming his scenes in Cardiff, Wales.
He told RadioTimes.com: "We kept it a secret because people in Cardiff are very kind. Simm was there for quite a few weeks, he was actually filming on the streets and nobody saw him and he was in the studio ... We were so thrilled by the idea of it, we were delight because John loves 'Doctor Who' and he loves the whole world of it, so when he heard what Steven Moffatt is planning he was so excited. It was brilliant to have him on set - he's a real joy to have around."
The BBC had tried to keep Simm's return under wraps but Minchin is pleased the news got out as it has created a massive buzz for the programme's return, which sees Pearl Mackie as new companion Bill Potts.
He added: "When people heard Simm was back they were excited and they were happy, so I'd be a bit churlish to be complaining about that. One of the things about 'Doctor Who' is we're a show people like to find out news about, and that they like to get excited about and that's a good thing really. It'd be a bit mean-spirited to whinge about that. The story of Bill and the Doctor is going to take some very surprising turns things are gonna happen to the Doctor that have never happened to him before and Matt Lucas is going to surprise you as well in his character, what you see from him. And we're building to the most amazing finale ever."
'Doctor Who' returns on Saturday (15.04.17) at 7:30pm with 'The Pilot'.