Game!

OK.

A mild-mannered, uptight gentleman is engaged to be married to a young lady he knows because of his job. The wedding is set for the next day. He meets another young lady, also because of his job (but in a more tertiary sense), who promptly steals his car. They embark on a series of mad adventures together with a great cat, spend the night together, and wind up in love.

What movie have I just described? (Hint: This is not as easy as it looks.)

UPDATE: Laurie walked away with that almost instantly. The answer is Bringing Up Baby AND Who's That Girl. There are so many more similarities than I mentioned - it's kind of distressing that I never noticed until Saturday. The biggest difference seems to be that Susan isn't a convict. (There are loads of small differences, but not many in the actual basic stories.)