Do the actors gain any 'abilities' when they transform? Plus I presume that they do gain the necessary background knowledge. It would be awkward not recognising members of your family or close personal associates because their either been dropped from the film or look nothing like the actual character.

With regards to Peter Capaldi I did read that one reason he gave up the role was that he found it physically too demanding. So unless he gets time-lord resources, such as the 2nd heart and the fact he's functionally immortal, that would be a problem. Not to mention if he actually found himself facing daleks, cybermen and the like can he rely on them being as stupid and accommodating as they generally are on TV? Similarly would he know how to use the sonic screwdriver or how to speak multiple languages and know history across a broad range of ages and multiple galaxies.

Those points would be ever more true for actors playing characters with superhero type powers. A superman who can't fly and can be killed by a bullet or a car crash say? Or even a martial artist who falls over his own feet when he tries to fight. [Reminded of a Rowan Atkinson advert for a chocolate bar if any of them have ever seen it. Checking its an ad for Snickers].

Sarah Sutton waking up in Doctor Who as teenage Nyssa... Especially as Nyssa was implied to have bigger boobs than Sarah also being thrust from a nearly sixty year old body to an eighteen year old one (assuming Sarah wakes up inside the story "The Keeper of Traken") will throw her for a loop.