When it comes to motion-capture and animation, spit-balling in the recording session is sort of key to the process.
For example, Robin Williams famously improvised almost all of his dialogue in 'Aladdin' and the animators worked around them. While the same can't be necessarily said for 'Detective Pikachu', it's not far off the mark. In an interview with Kotaku, Reynolds laid out the process of how it works inside the recording booth.
"Motion capture is a unique experience in which you are alone and you throw a billion things up against the wall... It's up to somebody else to sift through it and figure out what they think is going to be applicable and what they feel they want to use." In fact, Reynolds postulated that he came up with at least "85 options for any one joke", with many of his jokes ending up on the floor.
To make it even more interesting, Reynolds seemed pretty certain there was enough left behind to produce an R-rated version of 'Detective Pikachu' - but hastened to add it'll never see the light of day.
Release the R-rated cut, you cowards. Give the people what they want - a foul-mouthed, angry Detective Pikachu. We made peace with the fact that Danny DeVito wasn't going to voice him.