Christopher Nolan and Cillian Murphy have worked in a lot of films together. And if you observe closely then you will see that after Michael Caine, if there is one actor Nolan wants in his films then it’s Cillian Murphy.
They Love to Work Together
Christopher Nolan and Cillian Murphy met in 2003 when Cillian auditioned to play Batman/Bruce Wayne in Batman Begins. Of course, Christian Bale ended up getting the role, but Cillian made a good impression and ended up playing one of the villains in the film, and had a cameo in each of the sequels.

Cillian once spoke about how he feels about not getting to play Batman:
” I don’t believe I was close to landing that role. The only actor who was right for that part at that time, in my estimation, was Christian Bale, and he absolutely smashed it. So, for me, it was just an experience, and then it turned into something else. It turned into that character, Scarecrow, and it turned into a working relationship with Chris. So, I think back very, very fondly on that time, but I never, ever, ever considered myself Bruce Wayne material.”
After the release of The Dark Knight in 2008, Christopher Nolan started working on Inception, which would give him a breather from Gotham City and time to grieve the loss of his friend Heath Ledger.

When casting the roles he chose Cillian once again, and because there was a scene where his character had a bag in his face, like his character in Batman Begins, they even have a running gag about it:
“Every time I see Chris or every time we talk about doing something, we have a laugh about whether or not I’m going to be wearing a bag over my head at some point in the story. So it is a gag that has developed. I don’t know why he insists on doing that. (Laughs.) But yeah, there was nothing on my head in Dunkirk.”
They’ve worked together in 5 films, and they are great friends and known exactly what the other person wants and needs them to do in front and behind the camera.
Cillian Murphy Is A Great Actor

Even though Cillian Murphy did not get the role of Batman, Nolan was mighty impressed with Murphy’s acting chops. That is the reason why even though he could not get Murphy as Batman, he still managed to add him in the films as Scarecrow. There is no doubt that Murphy is a phenomenal actor.
So even for different movies, Murphy justifies his selection with his diverse acting skills. There are few English actors who Nolan likes to work with on a regular basis like Tom Hardy, Kenneth Branagh etc and Murphy is definitely on that list.