Chatting to Metro, the actress went on to state that the dynamic between the two of them was “the heart of the show” and very important to giving it the success that keeps audiences tuning in.
In many ways, Karen is completely correct, it is very important for the lead actors in any show and especially Doctor Who to create an enjoyable back and forth for viewers to get drawn into. There have certainly been some TARDIS crew members that have not bonded entirely well with the Doctor, leading to viewers having a difficult time enjoying the Doctor Who experience. Of note (and this has nothing to do with the actors at all) the Sixth Doctor and Peri had a patchy relationship filled with arguments and cynical comments thrown back and forth at each other leading to a fall in ratings.
But chemistry is not everything, after all the show is so much more than just the actors who bring it to life so amazingly well. The writers, producers and remaining members of the cast and crew put so much effort every year to make sure that audiences get the best that Doctor Who can offer.
However, Matt and Karen have a wonderful friendship that does indeed sparkle both in front and behind the camera.