Doctor Who star Peter Capaldi has praised actress Pearl Mackie amid speculation about her role as companion on the show.
Reports have suggested that Mackie, 29, will leave after just one series as the Doctor's new sidekick Bill Potts.
Asked about working with Mackie, Capaldi, who is quitting after the forthcoming tenth series, said the actress "brought a whole new vigour and excitement to the role of the Doctor's companion".
While not giving away any hints as to his co-star's future, the 58-year-old said: "The companion (and Bill is a very good example of it) is sort of their own person.
"They tend to be characters who are fully formed and independent ... To be a companion in Doctor Who you have to be your own person. It doesn't really work if the companion is just an adjunct to the Doctor."
Executive producer Steven Moffat, who is also leaving the BBC One show, said Capaldi's "input" had been "massively important" in casting the new assistant.
"They are going to be a working unit for months - they are going to see more of each other than they see of their significant others when they're playing these parts, so you're practically marrying them," he said in publicity material for the next series.
He added: "You're casting a friendship."
Mackie recently revealed that her character would be the Time Lord's first openly gay companion.
Moffat said of her alter ego: "She has a different bunch of questions - what are the questions that a real person flung into the Doctor's life would ask?
"So I've set this challenge to all the writers - what is she going to ask him? The moment you open that up it starts to define her. Where is the toilet on the Tardis - that's a really reasonable question."
The BBC has played down the rumours of a new Doctor Who companion, saying: "We never reveal the fate of individual characters on the show. Viewers will have to tune in to find out.
"We are still filming series 10 and no casting decisions have yet been made on series 11."
Moffatt, who is being replaced by Chris Chibnall, said the opening episode of the new series would be "Doctor Who at its purest".
Some episodes of Doctor Who have been criticised for being too complex.
The writer said of the first new episode: "The idea was just to introduce Doctor Who properly - the story starts here. You need to know nothing before this point."
He added: "The fact that this was my last series had to be removed from the mix. The fact that this is Peter's last series matters to the show."
Fleabag actress Phoebe Waller-Bridge, ex-Homeland star David Harewood and former Death In Paradise actor Kris Marshall are among the names tipped to take over from Capaldi in the Time Lord role.
:: Doctor Who returns to BBC One on Saturday April 15 with the first episode of series 10, The Pilot.