Bob Newhart

For anyone reading this all these years later and wondering the same thing, I assume you may have seen the film in the past and just came on imdb and noticed his name far down the cast list. It is due to his name being one of those found at the end of credits in the manner of "and featuring..." or "with..." coming before his name. Sometimes reserved for a bit part played by a big name actor but that's not the case here.

This movie was Bob Newhart's idea?

Newhart had acted in a few movies and TV shows before Cold Turkey, and he'd made his bestselling albums. But his best series, The Bob Newhart Show and Newhart, were in the future.

Meanwhile, Dick Van Dyke was a big name because of his eponymous TV series and Mary Poppins.

In short, Newhart had not yet earned higher billing. If the movie were being remade today, and he played the same role, he'd rank higher.