"I'm a pretty convenient foil for a lot of people"

In this quote, Kevin Costner is describing himself as a "convenient aluminum foil" for other people. An aluminum foil is a character in a story who contrasts with another personality in order to highlight particular top qualities of the various other personality. In this instance, Costner is recommending that he is often made use of as a contrast to other individuals in order to stress their qualities. This can indicate that he is frequently used as a contrast to demonstrate how another person is much better or worse than him. It might also indicate that he is made use of as a comparison to show how another person is different from him. In either instance, Costner is suggesting that he is typically utilized as a tool to highlight the high qualities of other people. This might be a positive or adverse thing, depending upon the context. Eventually, Costner is recommending that he is often utilized as a device to emphasize the high qualities of other people, whether it declare or negative.