I think this may depends a lot on the guest - search youtube for clips of Norm Macdonald on Conan. Then again, maybe Norm is just that good at faking it?
I think Conan, with his current show, has more freedom to let his guest go off script which helps with the spontaneity.
Fallon and Colbert are pretty controlled probably due to their mainstream audience - and advertisers.
It’s interesting to contrast Bill Burr’s appearance on all 3 shows.
Burr is most at home on Conan and worst on Colbert - who ran the interview like an interrogation; lost a lot of respect for Colbert after that (if you are going to be a wet blanket, might as well not book him).
On Fallon ... it’s hilarious to see Fallon fumble and panic as he tries to steer him towards the script then giving up half way.
I think this may depends a lot on the guest - search youtube for clips of Norm Macdonald on Conan. Then again, maybe Norm is just that good at faking it?