I think the phrase 'tennis ball theory' was coined by Jim Carrey and Tom Shadyac, but it's been in use since ancient Greece and beyond I'm sure. Basically, you have one crazy character and put him in a normal world, chaos insues. Think Ace Ventura, as that's probably the best example in cinema history. In that movie, every single character is a normal, realistic character except Ace, who is beyond insane. You set up the supporting characters to be strong, like a wall, and then the crazy guy is the tennis ball. The harder the wall, the harder the bounce.

In 'Entertaining Angels', I struggled a lot with the fact that there are a number of crazy characters who populate the film. Will they cannibalize each others laughs? Will it get so unbelievable that the humor will be lost, and worse, the film be a bore? Also, who does the audience sympathize with? Jack or Kitsy? If the audience doesn't know, the laughs won't be genuine. 'Mouse Hunt' is the best example of this, because while the craziest stuff is happening on screen, the audience is confused as whether to sympathize with the mouse or the people. I think I've come to the conclusion that it's not a big problem for several reasons.
Kate is the emotional anchor of the film, and a good one. People will relate to her, and she brings the movie down to earth.
The zany characters aren't similar to each other. In other words, Jack isn't the same kind of wacky that Kitsy is, and Kitsy isn't the same kind of wacky that the Professor is. So long as the teams stay Abbott and Costello and not Abbott and Abbott we should be okay.
In other movies, such as the great 'Bowfinger', every single character is insane and it works brilliantly. To successfully employ this type of screwball comedy, it takes a little absurdity. Also, the audience will be by default sympathize with Jack, because he's a sympathetic character. Kitsy doesn't need any sympathy, he can just be plain quirky and succeed.

We'll see soon if I'm right. I hope I am.